Summary: Allow splitting package.json to public one and fb-internal one located in "fb/package.json". When plugin is packaged, fields in package.json are overwritten by fields from "fb/package.json" if they exist. This give us a way to specify additional metadata which only make sense internally (e.g. oncall and internal links to docs and support).
Reviewed By: mweststrate
Differential Revision: D28542101
fbshipit-source-id: c0167461897a994e5731aaf0fe625de052eda864