Misleading error message for "use self::submodule"  when "pub mod submodule;" is in the same file

Example:

top.rs

```
use self::submodule;  // "error: unresolved import use self::submodule;"
pub mod submodule;

pub type Dummy = ();
```

and submodule.rs in the same directory

```
pub type Dummy2 = ();
```

The "use self::submodule" is declaring the same item name as "pub mod submodule" and should be illegal. The error message is misleading and needs to be changed.
