rustc: Suggest removing extern crate in 2018

[alexcrichton]

This commit updates the unused_extern_crates lint to make automatic
suggestions about removing extern crate annotations in the 2018 edition. This
ended up being a little easier than originally though due to what's likely been
fixed issues in the resolver!

Closes#52829

[rust-highfive]

r? @oli-obk

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[Mark-Simulacrum]

Could we add a test for when the crate is used only through macros, both in the use core::some_macro and the some_macro! with no import sense? I believe that some code at least handles that case separately, and recall different error messages, so would be good to check that we don't emit overlapping lints or anything like that.

[oli-obk]

r=me with @Mark-Simulacrum 's tests

[alexcrichton]

Good idea @Mark-Simulacrum! While the resulting code isn't fantastic, it's reasonable in the sense it produces no warnings. I'll file a follow-up issue.

@bors: r=oli-obk

[bors]

📌 Commit 51fd3bf has been approved by oli-obk

[alexcrichton]

I've filed #53741 as a follow-up

[bors]

⌛ Testing commit 51fd3bf with merge f4e981c...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: oli-obk
Pushing f4e981c to master...