Move fields functions out of the way
Notes:

- The only usage in `tslib` of `get_date_name_field` is for `Timestamp.weekday` and can be simplified with a more direct call.
- Several of the functions from `tslibs.fields` imported into `tslib` are unused.  Updating the external imports to use the new locations is for a follow-up.
- There is some flake8 cleanup to be done in the the new `tslibs.fields`, but that is being saved for a follow-up.  For now this is just cut/paste.
-  `get_start_end_field` will be largely unnecessary if/when the suggestion to deprecate Timestamp.freq in #15146 is adopted.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for updating the PR.



 - In the file [`setup.py`](https://github.com/pandas-dev/pandas/blob/0e9e1e5c8ee6950df80e542d8d6bacf2487d8af2/setup.py), following are the PEP8 issues :

> [Line 492:80](https://github.com/pandas-dev/pandas/blob/0e9e1e5c8ee6950df80e542d8d6bacf2487d8af2/setup.py#L492): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (82 > 79 characters)
> [Line 493:80](https://github.com/pandas-dev/pandas/blob/0e9e1e5c8ee6950df80e542d8d6bacf2487d8af2/setup.py#L493): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (92 > 79 characters)



##### Comment last updated on October 04, 2017 at 14:36 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=h1) Report
> Merging [#17770](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81694dce171ecd93a65e32ed455612ee967d3951?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17770/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17770      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49872    49872              
==========================================
- Hits        45514    45505       -9     
- Misses       4358     4367       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=footer). Last update [81694dc...7b12b5a](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=h1) Report
> Merging [#17770](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81694dce171ecd93a65e32ed455612ee967d3951?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17770/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17770      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49872    49872              
==========================================
- Hits        45514    45505       -9     
- Misses       4358     4367       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=footer). Last update [81694dc...7b12b5a](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=h1) Report
> Merging [#17770](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81694dce171ecd93a65e32ed455612ee967d3951?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17770/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17770      +/-   ##
==========================================
- Coverage   91.26%   91.22%   -0.04%     
==========================================
  Files         163      163              
  Lines       49872    49917      +45     
==========================================
+ Hits        45514    45536      +22     
- Misses       4358     4381      +23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <100%> (-0.02%)` | :arrow_down: |
| #single | `40.24% <50%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.78% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93% <0%> (-0.98%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.1% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.42% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=footer). Last update [81694dc...0e9e1e5](https://codecov.io/gh/pandas-dev/pandas/pull/17770?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > get_start_end_field will be largely unnecessary if/when the suggestion to deprecate Timestamp.freq in #15146 is adopted.

I think this would be a difficult change FYI

 thanks!