Unused imports sometimes reports used imports

One example, minimized from something in rustpkg:

```
extern mod extra(name = "extra");

use extra::getopts::groups::getopts;
use extra::getopts;

fn main() { getopts([], []); }
```

It reports that the first import is unused. If that import is removed, compilation fails. It is actually the _second_ import that is unused.
