CLN: reorg sorting algos 
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 
