CLN: Use exclusions in determination of _selected_obj
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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.
Possibly related #46944  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` Thanks @rhshadrach 