Summary:
This is the new getdeps entrypoint.  It lives in `opensource/fbcode_builder` so that
it is synced into the various opensource projects at FB.

In the incarnation in this diff is has a single subcommand that can be used to validate
a manifest file.

Reviewed By: sinancepel

Differential Revision: D14690995

fbshipit-source-id: 53f8ec9bafb7b1930970079e3ce20f496ac1c188