Auto merge of #53988 - eddyb:issue-53770, r=petrochenkov

rustc_resolve: only prepend CrateRoot to a non-keyword segment.

Fixes #53770 by treating `use` paths as absolute in a finer-grained manner, specifically:
```rust
use {a, crate::b, self::c, super::d};
```
Used to be interpreted as if it were (when `uniform_paths` is not enabled):
```rust
use ::{a, crate::b, self::c, super::d};
```
With this PR, the `CrateRoot` pseudo-keyword indicating an absolute path is only inserted when the first path segment is found (if it's not a keyword), i.e. the example behaves like:
```rust
use {::a, crate::b, self::c, super::d};
```
This should (finally) make `use {path};` fully equivalent to `use path;`.

r? @petrochenkov cc @cramertj @joshtriplett @nikomatsakis

src/test/ui/run-pass/rfcs/rfc-2126-crate-paths/crate-path-absolute.rs

@@ -23,8 +23,7 @@ mod m {
     pub(in crate::m) struct S;
 }
 
-mod n
-{
+mod n {
     use crate::m::f;
     use crate as root;
     pub fn check() {
@@ -34,9 +33,20 @@ mod n
     }
 }
 
+mod p {
+    use {super::f, crate::m::g, self::root::m::h};
+    use crate as root;
+    pub fn check() {
+        assert_eq!(f(), 1);
+        assert_eq!(g(), 2);
+        assert_eq!(h(), 3);
+    }
+}
+
 fn main() {
     assert_eq!(f(), 1);
     assert_eq!(crate::m::g(), 2);
     assert_eq!(root::m::h(), 3);
     n::check();
+    p::check();
 }