Move `PhutilLibraryMapBuilder` into the standard directory structure

Summary: See D9818.

Test Plan: Made sure `arc liberate` still works.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9822