cleanup inconsistently used imports
There are a bunch of modules that will do both `import pandas.core.common as com` and `from pandas.core.common import _whatever`, then sprinkled throughout we'll see both `_whatever` and `com._whatever`.  This PR tracks down a bunch of those and standardizes on `com._whatever`.

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

Hello @jbrockmendel! Thanks for submitting the PR.





 - In the file [`pandas/core/generic.py`](https://github.com/pandas-dev/pandas/blob/b6cb0557f74f0f6fb8458e07ed7e7af82af7c102/pandas/core/generic.py), following are the PEP8 issues :

> [Line 6358:80](https://github.com/pandas-dev/pandas/blob/b6cb0557f74f0f6fb8458e07ed7e7af82af7c102/pandas/core/generic.py#L6358): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (80 > 79 characters)













 I have no problem with doing this, but pls add a lint rule for this, otherwise over time it will just change again. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19292?src=pr&el=h1) Report
> Merging [#19292](https://codecov.io/gh/pandas-dev/pandas/pull/19292?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f0cd23ca6485940e3a6572cc4f5afdce4c3921ab?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `82.93%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19292/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19292?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19292      +/-   ##
==========================================
- Coverage   91.53%    91.5%   -0.03%     
==========================================
  Files         150      150              
  Lines       48738    48727      -11     
==========================================
- Hits        44611    44588      -23     
- Misses       4127     4139      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <82.53%> (-0.03%)` | :arrow_down: |
| #single | `41.71% <39.68%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19292?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.1% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/computation/expressions.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwcmVzc2lvbnMucHk=) | `93.27% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.51% <100%> (ø)` | :arrow_up: |
| [pandas/core/computation/align.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYWxpZ24ucHk=) | `97.89% <100%> (ø)` | :arrow_up: |
| [pandas/io/json/table\_schema.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vdGFibGVfc2NoZW1hLnB5) | `98.29% <100%> (ø)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.53% <100%> (ø)` | :arrow_up: |
| [pandas/core/nanops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbmFub3BzLnB5) | `96.68% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `92.62% <100%> (-0.01%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.45% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.23% <100%> (-0.01%)` | :arrow_down: |
| ... and [32 more](https://codecov.io/gh/pandas-dev/pandas/pull/19292/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19292?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/19292?src=pr&el=footer). Last update [f0cd23c...7872f33](https://codecov.io/gh/pandas-dev/pandas/pull/19292?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. generally making things consistent is good. however import format is almost a style thing and can be somewhat arbitrary and is hard to enforce. so limit these type of PR's unless there is really a massive difference or dichotomy across the codebase (which as true for common) @jbrockmendel how attached are we to importing from `pandas.core.common` with the `com` alias?

Because in there is the argument `com`

https://github.com/pandas-dev/pandas/blob/1f6e44a2354fa53b724eadb2c52ee84b02300500/pandas/core/generic.py#L11523-L11527

which shadows the import

https://github.com/pandas-dev/pandas/blob/1f6e44a2354fa53b724eadb2c52ee84b02300500/pandas/core/generic.py#L133-L136

---

If it's OK, I'd just import that one without an alias and `# noqa` it, as local variables shadowing global ones can potentially introduce bugs i dont feel strongly about it