2018 idioms: rustc-internal crates linted to be removed when they can't

[alexcrichton]

Discovered in rust-lang/cargo#6323

#![feature(extern_crate_item_prelude)]
#![feature(rustc_private)]
#![warn(rust_2018_idioms)]

extern crate syntax;

#[allow(unused_imports)]
use syntax::ast;

fn main() {}

yields:

warning: unused extern crate
 --> src/main.rs:5:1
  |
5 | extern crate syntax;
  | ^^^^^^^^^^^^^^^^^^^^ help: remove it
  |
note: lint level defined here
 --> src/main.rs:3:9
  |
3 | #![warn(rust_2018_idioms)]
  |         ^^^^^^^^^^^^^^^^
  = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]

This may be fixed by #55884, but it's not clear to me at least

[petrochenkov]

This doesn't reproduce on the current nightly, so apparently it was fixed by #56042.

[petrochenkov]

(Reopening as E-needstest.)