Since the interface generator is invoked via a Bazel macro, it will be
expanded in the embedding context of the project using SAPI, so package access
needs to go through the full workspace root @com_google_sandboxed_api.
This change also modifies the CMakeLists.txt accordingly, as the "external"
subdirectory is no longer needed/wanted.
PiperOrigin-RevId: 255918784
Change-Id: I052c687509f65fef7f011a9d1a074a171595330f
