REF: move _convert_to_indexer to Loc
It is overriden in iLoc, so by putting it directly in Loc we can simplify bits of it that check `self.name` and keep only the `self.name == "loc"` branches.  This lets us get rid of a bit of dead code (L1638-L1642).

Most of the diff is misleading; it is only the one method that is moved, with a placeholder kept in NDFrameIndexer