CLN: Use exclusions in determination of _selected_obj

[rhshadrach]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Toward #46944.

Currently we use whether a grouping is in-axis or not to determine if it should make it into _selected_obj. This is almost the same as using exclusions, except for a TimeGrouper. Here, the grouping data may arise from a column and hence should be excluded, however the grouping data is not the column itself but rather derived from the column, so it is not technically in-axis. Such columns should not make it into _selected_obj, so we should prefer using excluded directly rather than in-axis.

This does not change the behavior of Resampler because Resampler is a subclass of BaseGroupBy which does not have the selection context.

[jbrockmendel]

Possibly related #46944

[rhshadrach]

Right - some of the next steps include moving the group selection context to BaseGroupBy and using _selected_obj within the context in Resampler instead of _obj_with_exclusions

[mroeschke]

Thanks @rhshadrach