Add a "concrete only" flag to PhutilSymbolLoader

Summary: We have a few cases where we only want to load concrete subclasses.
Currently, we have the same ReflectionClass block copy/pasted a few places;
simplify it by putting the logic in PhutilSymbolLoader.

Test Plan: Modified Drydock, Conduit and phd to use setConcreteOnly(), ran "phd
list", visited conduit API console, hit Drydock "allocate resource" endpoint,
saw correct behavior.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T801

Differential Revision: https://secure.phabricator.com/D1486