Auto merge of #54005 - eddyb:uniform-paths-self-resolve, r=petrochenkov

rustc_resolve: allow `use crate_name;` under `uniform_paths`.

Specifically, `use crate_name;` and `use crate_name::{self, ...};` are now allowed, whereas previously there would produce a (false positive) ambiguity error, as the ambiguity detection code was seeing the `crate_name` import as a locally-available definition to conflict with the crate.

r? @petrochenkov cc @aturon @joshtriplett @Centril

src/test/ui/rust-2018/uniform-paths/redundant.rs

@@ -1,17 +0,0 @@
-// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// edition:2018
-
-#![feature(uniform_paths)]
-
-use std;
-
-fn main() {}

src/test/ui/rust-2018/uniform-paths/redundant.stderr

@@ -1,14 +0,0 @@
-error: `std` import is redundant
-  --> $DIR/redundant.rs:15:5
-   |
-LL | use std;
-   |     ^^^
-   |     |
-   |     refers to external crate `::std`
-   |     defines `self::std`, shadowing itself
-   |
-   = help: remove or write `::std` explicitly instead
-   = note: relative `use` paths enabled by `#![feature(uniform_paths)]`
-
-error: aborting due to previous error
-