CLN: reorg sorting algos

[jreback]

currently much of the sorting algorithms are in groupby.py (e.g. _index_from_factorized, _lexsort_indexer), the whole section under 'misc utilities'.

I think it makes sense to move these to algorithms.py. (OR maybe to sorting.py). Likewise might make sense to pull out 'sorting' type cython algos and to add a section / extension (or at least group the code a bit more).

xref #9019

[jreback]

cc @behzadnouri
cc @immerrr
@shoyer

[jreback]

closed by #15402