Fix build-plugin script
Fix build-plugin script

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

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper