edition compatibility: No warning emitted for crate/macro of the same name

[alexcrichton]

Originally reported upstream this code:

#![warn(rust_2018_compatibility)]
#![allow(unused_imports)]

#[macro_use]
extern crate toml; // 0.4.8

mod x {
    use toml;
}

when compiled emits no warnings, but when compiled in the 2018 edition fails to compile.

This appears to have something to do with the fact that the toml crate has a toml macro perhaps? It seems unclear!