Avoid circular import between core.series and plotting._xyz
This follows up on #16913 but cuts down on the scope of the PR.

`pandas.core.series` currently runs

`import pandas.plotting._core as _gfx  # noqa`

about 8 lines from the bottom of the file.  `pandas.plotting._core` runs

`from pandas.core.series import Series, remove_na` 

at the top of the file.  Using very small words and talking very slowly, it was explained to me that this circular import works because `Series` and `remove_na` are defined in the `series` namespace by the time `plotting._core` tries to import them.

This PR gets rid of the module-level import from `core.series` and moves the import of `plotting._core` to the top of `core.series` so as to avoid relying on hard-to-predict/debug behavior.

Other changes that I'd like to make (like deleting the extraneous `ret = Series()` at plotting/_core.py:L2325) are ignored for now.

 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
