PERF/REF: make window/groupby imports lazy
Maybe makes 20ms difference, hard to measure.  But separating this out from other import-speed stuff bc the docstring movement makes the diff big.
Restored groupby imports in core.api bc i think dask accesses `pd.core.groupby.GroupBy`.  Restoring that basically undoes the perf improvement, so this is just a refactor at this point