Summary: Right now the entire RN core code lives inside one giant internal Buck target. This makes it hard to refactor the infra and to roll out TurboModules. For now, create a baseline for how RN core dir can be structured.
Reviewed By: PeteTheHeat
Differential Revision: D16001260
fbshipit-source-id: bba947e2fb75576a2e1f3f4c816575f1157dcb03