Summary:
Update `BuildOpts.compute_dirs()` to use the correct project-specific manifest
context when computing project hashes.  Previously it was incorrectly using
the initial project's context when evaluating all dependencies.  This would
result in some projects potentially seeing the wrong values for variables that
may change from project to project (like `test`).

Reviewed By: pkaush

Differential Revision: D16477398

fbshipit-source-id: 6c23f5e5e19b2402000a138b3920b79044446041