locator: Move Git-specific knowledge into localrepo interface

The locator interface is currently hosting some functions which contain Git-specific knowldge about the repository layout. This is the wrong place to have this knowledge though: the locator should be able to locate Gitaly-specific items like the storage paths or specific repositories, but it shouldn't know anything about the Git repos themselves.

This MR moves functions which return Git-specific paths into the localrepo interface instead.

Part of #2721 (closed)