Move build-time resources to "CelerityPhysicalResources" to fix Phame

Summary:
Ref T4222. This fixes some issues with Phame's resource construction.

Phame requires a fully virtual resource source, and since I want to run wordpress templates unmodified some day I don't want to build resource maps for skins.

Move all the stuff that depends on resource lists being discoverable at build time to CelerityPhysicalResources, and only generate maps for subclasses.

The root CelerityResources can now construct virtual resources; construct a virtual resource for Phame and use it.

Test Plan: Off-domain blogs work correctly now. On-domain blogs with custom skins work correctly now.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7873