std/rustc/rustpkg/syntax: Support the extern mod = ... form

This commit allows you to write:

 extern mod x = "a/b/c";

which means rustc will search in the RUST_PATH for a package with
ID a/b/c, and bind it to the name `x` if it's found.

Incidentally, move get_relative_to from back::rpath into std::path

src/libsyntax/ast_util.rs

@@ -419,7 +419,7 @@ impl Visitor<()> for IdVisitor {
 
     fn visit_view_item(@mut self, view_item: &view_item, env: ()) {
         match view_item.node {
-            view_item_extern_mod(_, _, node_id) => {
+            view_item_extern_mod(_, _, _, node_id) => {
                 (self.visit_callback)(node_id)
             }
             view_item_use(ref view_paths) => {