Improve unresolved use error message
"use of undeclared type or module `foo`" doesn't mention that it could be a crate.
This error can happen when users forget to add a dependency to `Cargo.toml`, so I think it's important to mention that it could be a missing crate.
I've used a heuristic based on Rust's naming conventions. It complains about an unknown type if the ident starts with an upper-case letter, and crate or module otherwise. It seems to work very well. The expanded error help covers both an unknown type and a missing crate case.
