rust-matchers: implement `visit_children_set` for `FileMatcher` As per the removed inline comment, this will become useful in a future patch in this series as the `IncludeMatcher` is introduced. Differential Revision: https://phab.mercurial-scm.org/D7914

manifest: move matches method to be outside the interface In order to adequately smoke out any legacy consumers of the method, we rename it to _matches so it's clear that it's class-private. To my amazement, all consumers of this method really only wanted matching filenames, not a full filtered manifest. Differential Revision: https://phab.mercurial-scm.org/D8085