2018 idioms: incorrect suggestion for root module and crate of same name

[alexcrichton]

First reported at rust-lang/cargo#6273

this code:

#![warn(rust_2018_idioms)]

extern crate time as std_time;

pub mod time {
    pub fn f() {
        println!("{:?}", crate::std_time::now());
    }
}

fn main() {}

when compiled gives a warning that, when applied, produces code that doesn't compile

[alexcrichton]

@ehuss notes that this code is likely relevant here.