utils: move the dirs definition in pathutil (API)
Before this change, the dirs class was accessible through the mercurial.util
module. That module is expected to stay free of scm specific content.
The pathutil destination has been selection by Martin von Zweigbergk.
This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.
Differential Revision: https://phab.mercurial-scm.org/D7311