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