Lint against single-element `use` paths in 2018 edition

[cramertj]

I wind up leaving a lot of code review comments about not writing things like use std; or use some_extern_crate; in 2018 edition.

[petrochenkov]

This is an extension of the "redundant import" lint implemented in #58805.

Right now it covers imports in blocks

fn main() {
    use std; // warning: the item `std` is imported redundantly
}

but not in modules (due to some complexities described in #10178 (comment)).

[joshtriplett]

We discussed this in the @rust-lang/lang meeting. We agreed that 1) there should be a lint for this, and 2) ideally it should be a fixed version of the redundant import lint.

[kpreid]

This would be a good lint to have because many beginners speculate that use foo; brings foo into scope, as opposed to the actual situation that use foo::bar; brings bar into scope given that foo already is in scope. In my opinion, it would be best if

• The lint has help that specifically explains that 1-element paths in use usually do nothing and are only useful in special cases (pub use, as).
• The lint is deny-by-default in a future edition, because it indicates a misunderstanding of the module/crate/path system which is likely to result in follow-up errors (such as if they assume that use foo; works like use foo::*;), so the user should be promptly told “this won't help”. (Many beginners will completely ignore warnings while trying to fix an error.)

[petrochenkov]

This was implemented in #117772.
There was a large ecosystem fallout though, so #121708 now discusses how to release this change better.