rustc_ast_lowering: Stop lowering imports into multiple items

Lower them into a single item with multiple resolutions instead.
This also allows to remove additional `NodId`s and `DefId`s related to those additional items.

compiler/rustc_parse/src/parser/item.rs

@@ -1012,7 +1012,7 @@ impl<'a> Parser<'a> {
                     prefix.span = lo.to(self.prev_token.span);
                 }
 
-                UseTreeKind::Simple(self.parse_rename()?, DUMMY_NODE_ID, DUMMY_NODE_ID)
+                UseTreeKind::Simple(self.parse_rename()?)
             }
         };