Auto merge of #54011 - eddyb:anchored-in-the-future, r=petrochenkov

Auto merge of #54011 - eddyb:anchored-in-the-future, r=petrochenkov

rustc_resolve: inject `uniform_paths` canaries regardless of the feature-gate, on Rust 2018.

This PR is an attempt at future-proofing "anchored paths" by emitting the same ambiguity errors that `#![feature(uniform_paths)]` would, with slightly changed phrasing (see added UI tests).

Also, on top of #54005, this PR allows this as well:
```rust
use crate_name;
use crate_name::foo;
```
In that any ambiguity between an extern crate and an import *of that same crate* is ignored.

r? @petrochenkov cc @aturon @Centril @joshtriplett