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