REF: groupby internal names

[rhshadrach]

I'd like to start gathering / discussing refactoring some of the naming within pandas.core.groupby. These are some initial ones that come to mind, I plan to expand this issue in the near future.

Issues:

• (DEPR: pandas.core for groupby #55429 (comment)) generic.py has subclasses, whereas core.generic.py has the base class NDFrame.
• result_index is only the result index for aggregations.
• Is the 'b' capitalized in class names? DataFrameGroupBy vs ExpandingGroupby

Resolutions:

[rhshadrach]

For generic.py, I'd like to split this into one for SeriesGroupBy and one for DataFrameGroupBy. This would make it easier to determine what method you're looking at (the Series version vs the DataFrame version). I propose either calling them series.py and frame.py or series_groupby.py and frame_groupby.py.

For result_index, I'd like to rename this agg_index.

cc @jbrockmendel

[jbrockmendel]

For result_index, I'd like to rename this agg_index.

fine by me