unused_extern_crates warns about extern crates that only supply macros

[joshtriplett]

I ran into this when using quick_error. Minimal test case (along with a Cargo.toml including quick-error="*" in [dependencies]):

#![warn(unused_extern_crates)]

#[macro_use]
extern crate quick_error;

quick_error! {
    #[derive(Debug)]
    enum Error {
        Msg(msg: String) {
            from()
            from(s: &'static str) -> (s.to_string())
            description(msg)
            display("{}", msg)
        }
}

This produces:

src/main.rs:4:1: 4:26 warning: unused extern crate
src/main.rs:4 extern crate quick_error;
              ^~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:1:9: 1:29 note: lint level defined here
src/main.rs:1 #![warn(unused_extern_crates)]
                      ^~~~~~~~~~~~~~~~~~~~

[sanxiyn]

Closing as a duplicate of #30849. Thanks!