Move helper methods for Named Aggregation out of groupby/generic

[christopherzimmerman]

Related to #28333 and #26513

There are several helper methods to help with named aggregations, that are found in pandas.core.groupby.generic, namely:

• _is_multi_agg_with_relabel
• _normalize_keyword_aggregation
• _make_unique

If this functionality gets standardized across aggregations other than just groupby, it would make sense to have these moved out of this file.

[WillAyd]

Makes sense. I think would take a PR if you have something concrete to propose

@TomAugspurger

[TomAugspurger]

Yeah. I'd recommend making a branch that does everything (moving the helpers, implementing NamedAgg for window), to prove things work, and then split the moving of helpers to its own PR.

[jbrockmendel]

These now live in core.apply, closing as complete.