Support loading v2 libraries in libphutil

Summary:

• When loading a v2 library, load all its functions.
• When loading a library, build the classtree at load time instead of when we run a PhutilSymbolLoader query with setAncestorClass. This is slightly less efficient for the common case (zero calls), but simpler.
• Ignore phutil_require_module() calls that load v2 library modules, so a v1 library can continue to reference a v2 library.
• Load symbols in v2 libraries by loading the appropriate files.

Test Plan: Converted libphutil, arcanist and phabricator to v2, one at a time. Loaded stuff in Phabricator.

Reviewers: vrana, nh, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1103

Differential Revision: https://secure.phabricator.com/D2587