Use PhutilClassMapQuery instead of PhutilSymbolLoader
Summary: Use PhutilClassMapQuery instead of PhutilSymbolLoader, mostly for consistency.
Test Plan: N/A
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D13571