Summary:
Add a new ManifestLoader class to handle loading manifests and computing
dependencies.

For now the main thing this class does is maintain the `manifest_by_name`
mapping.  In subsequent diffs we should be able to move some additional logic
into this class, which will help clean up the code and eliminate some redudant
work.  In particular, we can have this class cache project hashes, which will
avoid re-computing hashes over and over again for the same projects as we do
in many cases today.  We should also be able to save and re-use some of the
project dependency ordering computation in some cases as well.

Reviewed By: strager

Differential Revision: D16477400

fbshipit-source-id: f06f62f77d8443fccaa69fe4c1306e39c395b325