utils: move finddirs() to pathutil This is a follow-up to c21aca51b392 (utils: move the `dirs` definition in pathutil (API), 2019-11-06). finddirs() is closely related to dirs and used by it. Differential Revision: https://phab.mercurial-scm.org/D7388

#require black

  $ cd $RUNTESTDIR/..
  $ black --config=black.toml --check --diff `hg files 'set:**.py - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`