This updates the Flutter tools to match the proposed new packaging of artifacts
in the engine release script.
* The GCS URL for artifacts is now gs://mojo/flutter/$revision/$platform
* Categories have been removed from the Artifact class
* All artifacts for a given platform now live in a zip file.  If an artifact
  is not present in the local cache, then the zip will be downloaded and
  extracted.

Note that darwin-x64 artifacts go through a different process that (for now)
continues to use the old format.