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 emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support
Ran 1 tests, 1 results as expected (*) (glob)