Unadorned path should resolve same as relative path written w.r.t. self/super

The following test should have all imports as valid:

``` rust
use a;        // bad
use self::a;  // good
use a::b;     // good

mod a {
  pub type b = int;
}
mod b {
  use a;        // bad
  use super::a; // good
  use a::b;     // good
}
```

Apparently resolve isn't realizing that `use a` is equivalent to `use super::a`. A little more discussion can be found in #6226 
