Missed unused import in 2018 edition when importing a crate

[alexcrichton]

Code like this:

fn main() {
    test1::foo();
    test2::foo();
}

mod test1 {
    use std;
    
    pub fn foo() {
        std::mem::drop(3);
    }
}

mod test2 {
    use std;
    
    pub fn foo() {
        std::println!("x");
    }
}

when compiled yields no warnings currently. In theory, though, both imports of std are unused due to the 2018 crate prelude!