libextra: Rename the actual metadata names of libcore to libstd and libstd to libextra

src/libsyntax/ext/source_util.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 codemap;
 use codemap::{FileMap, Loc, Pos, ExpandedFrom, span};