Interaction with uniform_paths canaries can hide import errors and lead to strange ICEs.

[eddyb]

Reduced from RLS' source (on the Rust 2018 edition):

// Dummy import to introduce `uniform_paths` canaries.
use std;

// fn version() -> &'static str {""}

mod foo {
    // Error *not* reported, despite `version` being commented out above!
    use crate::version; 
    
    fn bar() {
        version();
    }
}

fn main() {}

The use std; import is the one introducing the canaries - if it's changed to use ::std;, the error for version is emitted. With no import errors, various parts of the compiler can ICE.
(currently attempted .def_id() on invalid def: Err, triggered by lints trying to run on foo::bar)

I'll try to fix it during #54116, as it was discovered during my attempt to fix RLS for that PR.