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