Replace absolute paths with relative ones

Modern compilers allow reaching external crates
like std or core via relative paths in modules
outside of lib.rs and main.rs.

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -20,7 +20,7 @@ pub struct StableHasher {
 }
 
 impl ::std::fmt::Debug for StableHasher {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{:?}", self.state)
     }
 }