Summary:
Move code that computes project hashes to ManifestLoader.  ManifestLoader is
the only class that has all of the information necessary to compute the
project hashes correctly.  The ManifestLoader object can also cache previously
computed hashes, so that we don't have to keep computing hashes for projects
over and over again.  Previously the `BuildOptions.compute_dirs()` function
would end up re-computing hashes for all dependencies each time it was called.

Reviewed By: strager

Differential Revision: D16477401

fbshipit-source-id: ce03642114f91ce4f859f612e6b2e747cf1653be