Unused imports lint should detect redundant prelude imports

[jonas-schievink]

This code produces no warning:

use std::vec::Vec;

fn main() {
    Vec::<u8>::new();
}

A warning would be useful for old code that imports items which have since made it into the prelude.

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!

[jonas-schievink]

This isn't just a bug?

[jonas-schievink]

FWIW, I still believe that this is just a regular enhancement of the unused_imports lint that doesn't require an RFC. Just like #33091.