Create second project for mypy-protobuf

[nipunn1313]

This was a rudimentary way of having multiple mypy commands
per project (as needed here). It might be more elegant to have
multiple mypy_cmd within a project, but it seemed nontrivial to
achieve, and I was able to whip this mechanism up more quickly -
using multiple projects with one mypy_cmd per project.
Not the most proud of it, but it works.

[nipunn1313]

It's not super critical this gets in.
If the implementation strategy gives you the eebie jeebies, I think it's ok for this to wait for a better impl.

[hauntsaninja]

Thanks, let me think about this one!