librustc: Enforce that extern mod directives come first, then use directives, then items.

Resolve them in this order as well.

src/test/auxiliary/extern_mod_ordering_lib.rs

@@ -0,0 +1,6 @@
+#[crate_type="lib"];
+
+pub mod extern_mod_ordering_lib {
+    pub fn f() {}
+}
+

src/test/run-pass/extern-mod-ordering-exe.rs

@@ -0,0 +1,11 @@
+// aux-build:extern_mod_ordering_lib.rs
+// xfail-fast
+
+extern mod extern_mod_ordering_lib;
+
+use extern_mod_ordering_lib::extern_mod_ordering_lib;
+
+fn main() {
+    extern_mod_ordering_lib::f();
+}
+