unused_extern_crates lint should handle aliasing

[nrc]

e.g., extern crate rls_span as span;. I'm not sure what the suggestion should be though = pub use ... as ...; to replace the extern crate?

[CryZe]

Last time I checked pub use doesn't actually reexport crates as reported here: #52140

[alexcrichton]

Some data points:

• As of today's nightly this construct emits a warning that indicates that a use statement should be used instead, but no rustfixable suggestion is emitted.
• However, precisely the same warning happens for a non-renamed crate as well (this was reported as #![warn(rust_2018_idioms)] proposes incorrect resolution to extern crate #52832)
• If the crate isn't actually used you get a warning to remove it
• When migrating 2015 code relying on extern crate renaming you get a suggestion to use crate:: which does indeed work

This all makes sense with today's implementation which only emits suggestions for 2018 code and doesn't emit machine-applicable suggestions.

Given all that I'm going to close this issue in favor of #52829. I think it's basically just a bug in the compiler today that we're not emitting rustfixable suggestions for removing extern crate annotations. Fixing #52829 will involve taking this issue into account, but otherwise I think that this is an issue currently about a hypothetical buggy implementation that doesn't implement handling renames, but an implementation doesn't even exist today!