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