Lift class map construction into a new class map query

Summary:
Ref T8637. The "load concrete subclasses" pattern has enjoyed wide success, but we're starting to collect a lot of instances of "loop over this list of things and make sure they all really have unique keys".

We can better support this, caching, subclass expansion, and sorting in a thin layer on top of PhutilSymbolLoader.

Test Plan: See next diff.

Reviewers: btrahan, joshuaspence

Reviewed By: btrahan, joshuaspence

Subscribers: epriestley

Maniphest Tasks: T8637

Differential Revision: https://secure.phabricator.com/D13391