Allow PhutilClassMapQuery to load implementors of an interface

Summary: Fixes T9179. The old way worked for interfaces, but the new way didn't; it should.

Test Plan: No more fatal on /flag/.

Reviewers: chad, joshuaspence

Reviewed By: joshuaspence

Maniphest Tasks: T9179

Differential Revision: https://secure.phabricator.com/D13898