Remove the hack that generates nano package names.

We should use a helper function from protobuf, which is more reliable, but not there yet (google/protobuf/issues/778)

Start by reading google/protobuf/issues/778 to determine whether the referenced helper now exists and what interface it provides. The issue names no repository file, test, or entry point; completion would require replacing the nano package-name hack with that helper once it is available and verifying the generated package names.

索引モデルが issue の本文から書いたものです。