Remove unused cimports
This helps a little bit with the complexity of the libs interdependencies.

Make properties.pyx its own module, since we will end up needing it to be separate when `tseries.offsets` gets moved into cython.

Remove the long-defunct _libs/src/offsets.pyx.

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

you are doing quite a lot of things here. specifically don't include the property changes here. properties separated out into #17590  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=h1) Report
> Merging [#17585](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/21a38008e3cab7a0459cce4fab4ace11379c3148?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17585/graphs/tree.svg?token=eZ4WkYLtcO&height=150&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17585      +/-   ##
==========================================
+ Coverage   91.19%    91.2%   +<.01%     
==========================================
  Files         163      163              
  Lines       49626    49625       -1     
==========================================
+ Hits        45258    45261       +3     
+ Misses       4368     4364       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.19% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-0.16%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.9% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.73% <0%> (+0.2%)` | :arrow_up: |
| [pandas/plotting/\_tools.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190b29scy5weQ==) | `79% <0%> (+6.07%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17585?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/17585?src=pr&el=footer). Last update [21a3800...764785e](https://codecov.io/gh/pandas-dev/pandas/pull/17585?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/17585?src=pr&el=h1) Report
> Merging [#17585](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d1fe892a754bf48839d9ac4029e258883ee64a2e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17585/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17585      +/-   ##
==========================================
+ Coverage   91.19%    91.2%   +<.01%     
==========================================
  Files         163      163              
  Lines       49652    49652              
==========================================
+ Hits        45282    45285       +3     
+ Misses       4370     4367       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.18% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `78% <0%> (+12%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17585?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/17585?src=pr&el=footer). Last update [d1fe892...eaa48da](https://codecov.io/gh/pandas-dev/pandas/pull/17585?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!