Merge BUCK file at Libraries/ into root

Summary:
Merges the Facebook-internal Buck target definitions in `Libraries/` into the BUCK file at the root of the repo (which is currently not synced to GitHub at all). This does not affect how React Native is built in open source.

Changelog: [Internal]

Reviewed By: GijsWeterings

Differential Revision: D27967499

fbshipit-source-id: 39c51a544b3868242598072d24cb6cfb5a6e2d8c

ReactAndroid/src/main/java/com/facebook/react/devsupport/BUCK

@@ -40,7 +40,7 @@ rn_android_library(
         react_native_target("res:devsupport"),
     ],
     exported_deps = [
-        react_native_root_target("Libraries:FBReactNativeSpec"),
+        react_native_root_target(":FBReactNativeSpec"),
     ],
 )