auto merge of #7325 : artagnon/rust/resolve-module, r=cmr

Fix #7322.

I started out with a band-aid approach to special-case the duplicate module error using `is_duplicate_module`, but thought this would be better in the long term.

src/test/compile-fail/issue-3099-b.rs

@@ -10,6 +10,6 @@
 
 pub mod a {}
 
-pub mod a {} //~ ERROR duplicate definition of type `a`
+pub mod a {} //~ ERROR duplicate definition of module `a`
 
 fn main() {}