Summary:
Check that all variable names are valid when loading manifest files.
This ensures that getdeps.py will error out if someone makes a typo in a
variable name, rather than treating it as never equal to anything.

Reviewed By: pkaush

Differential Revision: D16477397

fbshipit-source-id: 030e0642ff4a08db8eb74a0a0223e03d53e4880f