R1:368f3591142e

Use PhutilClassMapQuery instead of PhutilSymbolLoader

Summary: Use `PhutilClassMaQuery` instead of `PhutilSymbolLoader`, mostly for consistency. Depends on D13588.

Test Plan: Poked around a bunch of pages.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13589