unused_extern_crates: suggests removing sysroot crate when used inside a submodule

[ehuss]

The following gives a suggestion to remove the extern crate which results in broken code in 2018.

#![warn(unused_extern_crates)]

extern crate proc_macro;  // suggests removing this which is incorrect.

pub mod m {
    pub fn f(_ts: proc_macro::TokenStream) {
    }
}

The lint doesn't fire if the crate is used in the root module.

[ehuss]

This was fixed by #57557. @petrochenkov, if you're satisfied that the existing tests cover this, feel free to close this issue. extern-crate-used.rs from that PR seems to be the closest existing test, which seems pretty close to the same as this.