switch package to leverage the upstream content if it's available and only provide `generate_proto`

[tfoote]

This is as discussed here: https://github.com/ros/rosdistro/pull/19184#issuecomment-424827756

It's also been being tracked here: https://github.com/ros/rosdistro/issues/19295 but I'm going to close that in favor of this enhancement request since there's nothing we can do at the rosdistro level to resolve this.