Summary: This diff fixes "build-plugin" script which is failing for fb-internal plugins, because "fb/package.json" not exist. Because of that release of "fb/mobilebuilds" plugin failed recently: https://www.internalfb.com/intern/sandcastle/job/13510799309874638.
Reviewed By: passy
Differential Revision: D28624924
fbshipit-source-id: e00da98cfef992f5d49045afc9d404764a0d13ac