Remove PhutilSymbolLoader::loadClass(), add setPathPrefix()

Summary:

• There are no more callsites to loadClass(); get rid of it.
• Add setPathPrefix() to replace setModule(), we need this for unit tests.

Test Plan: Fixed unit tests for v2. Grepped for loadClass().

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1103

Differential Revision: https://secure.phabricator.com/D2621