Use PhutilClassMapQuery instead of PhutilSymbolLoader
Summary: Use PhutilClassMapQuery instead of PhutilSymbolLoader, mainly for consistency. Depends on D13572.
Test Plan: This hasn't been tested very comphrehensively as of yet.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D13573