rust-utils: use new find_dirs iterator In cad3dde7a573, the `find_dirs` util was introduced, but the second changeset that made use of it didn't apply. This change fixes the issue. Differential Revision: https://phab.mercurial-scm.org/D6639

from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)