Cassaundra

Thoughts I had from a discussion with Ed: My inclination here is to have `cargo install` still error, but provide a more useful suggestion. I see a couple of problems...

@rustbot claim

Implemented in #12078. Maybe close?

I'd like to know more about the role of static checks in this hypothetical system; some examples would be nice. Relatively trivial checks like inspections of `Cargo.toml` or `cargo audit`...