Summary:
Treat only packages with "flipper-plugin" keyword as Flipper plugins. This will allow to place lib packages re-used by different plugins into "plugins" folder.
Changelog: Internals: it is now possible to add modules for re-use by different plugins into `desktop/plugins` folder.
Reviewed By: mweststrate
Differential Revision: D20898133
fbshipit-source-id: 8934870350ce42af3dc8060d1494025519ed307e