REF: Change how pandas.core.common shim works
I have a PR incoming that would introduce a circular dependency
between pandas.core.arrays.categorical, pandas.api, and pandas.core.common.

This change will allow that PR to avoid hacky workarounds, since the
real problem is pandas.core importing pandas.api.types.

xref https://github.com/pandas-dev/pandas/pull/13990
these are on the list to be removed in 0.23 anyhow( https://github.com/pandas-dev/pandas/issues/6581). would rather just take them out. Hmm, looks like dask uses it in one place :/

Thoughts up bumping it to a FutureWarning for one more release and then removing? Ah, dask's usage is just compatibility for older pandas (0.19.2, which isn't so long ago). this should go now
it’s been a while # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19304?src=pr&el=h1) Report
> Merging [#19304](https://codecov.io/gh/pandas-dev/pandas/pull/19304?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ca2d2611c5a657720661990e2f3016356e577d1d?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19304/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19304?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19304      +/-   ##
==========================================
- Coverage   91.52%    91.5%   -0.03%     
==========================================
  Files         150      150              
  Lines       48875    48877       +2     
==========================================
- Hits        44733    44723      -10     
- Misses       4142     4154      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.66% <72.22%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19304?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19304/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `92.96% <100%> (+0.04%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19304/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19304?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/19304?src=pr&el=footer). Last update [ca2d261...7b6d0b0](https://codecov.io/gh/pandas-dev/pandas/pull/19304?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 as I said above, just remove the core.common accessors I think we're undecided on what to do with deprecations slated for 0.23.0, comment from @jorisvandenbossche in https://github.com/pandas-dev/pandas/issues/6581#issuecomment-357449108. Let's resolve that in #6581 first. this was an already slated deprecation for removal 