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_hir/src/definitions.rs

@@ -118,7 +118,7 @@ impl DefKey {
 
         let DisambiguatedDefPathData { ref data, disambiguator } = self.disambiguated_data;
 
-        ::std::mem::discriminant(data).hash(&mut hasher);
+        std::mem::discriminant(data).hash(&mut hasher);
         if let Some(name) = data.get_opt_name() {
             // Get a stable hash by considering the symbol chars rather than
             // the symbol index.