Stabilize crate_in_paths, extern_absolute_paths and extern_prelude on all editions.

src/librustdoc/core.rs

@@ -455,12 +455,10 @@ pub fn run_core(search_paths: SearchPaths,
                                                         |_| Ok(()));
         let driver::InnerExpansionResult {
             mut hir_forest,
-            mut resolver,
+            resolver,
             ..
         } = abort_on_err(result, &sess);
 
-        resolver.ignore_extern_prelude_feature = true;
-
         // We need to hold on to the complete resolver, so we clone everything
         // for the analysis passes to use. Suboptimal, but necessary in the
         // current architecture.