Fix error message for duplicate mods

Code like

``` rust
mod foo {
    struct S;
}
mod foo {
    struct P;
}
```

Generates the error

```
foo.rs:4:0: 6:1 error: duplicate definition of type `foo`
foo.rs:4 mod foo {
foo.rs:5     struct P;
foo.rs:6 }
foo.rs:1:0: 3:1 note: first definition of type foo here:
foo.rs:1 mod foo {
foo.rs:2     struct S;
foo.rs:3 }
```

Which is wrong. `foo` is not a type, it's a module.
