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