Outdated `--explain` examples for E0659 "An item usage is ambiguous."

[mattheww]

See E0659 for the current text.

It includes the following example:

pub mod moon {
    pub fn foo() {}
}

pub mod earth {
    pub fn foo() {}
}

mod collider {
    pub use moon::*;
    pub use earth::*;
}

fn main() {
    collider::foo(); // ERROR: `foo` is ambiguous
}

which needs updating for Rust 2018: it needs crate:: or parent:: adding inside mod collider.

The same is true in the recommended replacement.

[Centril]

cc @GuillaumeGomez