unused_extern_crates: suggests removing sysroot crate when used inside a submodule

The following gives a suggestion to remove the `extern crate` which results in broken code in 2018.

```rust
#![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.
