Summary:
This defines a separate codegen entrypoint for the entire react-native repo, picking up all JS files under Libraries/ subdir. Buck can then build the codegen library targets. This is a pre-req for removing the checked in Java spec files for NativeModules.
Changelog: [Internal]
Reviewed By: RSNara
Differential Revision: D24518569
fbshipit-source-id: 2ece53c7274698e2431a00e590fad213e3a0bece
