librustc: Make unqualified identifier searches terminate at the nearest module scope. r=tjc

src/libsyntax/ext/base.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::prelude::*;
+
 use ast;
 use ast_util::dummy_sp;
 use codemap;