flatten commands

The `usage` CLI has command structures that are way too deep considering it only has a handful of commands. e.g.: `usage generate completion bash` is very deep considering the only other command right now is `usage complete-word` which is only ever used internally.

Naively, this could just be `usage gen-completion bash` or something.

Before I do this I need to think a bit long-term and about what commands this CLI might contain.