Code review comment for ppa-dev-tools:fix-lp1993368-cli-arg-behavior

Thanks for the review, I incorporated most of the changes Friday and will land this branch today.

I wanted to think more about the suggestion for a helper routine for creating the subparsers, but I think I'm going to hold off on that and see how the code evolves first. (There's going to be a bunch of subparser help text, and a bunch of repetitive cli options added, which may suggest a different refactoring approach.)