By tracking import use types to check whether it is scope uses or the other situations like module-relative uses, we can do more accurate redundant import checking.

fixes rust-lang#117448

For example unnecessary imports in std::prelude that can be eliminated:

```rust
use std::option::Option::Some;//~ WARNING the item `Some` is imported redundantly
use std::option::Option::None; //~ WARNING the item `None` is imported redundantly
```

Author: surechen

std/src/os/unix/fs.rs

@@ -397,7 +397,6 @@ pub trait OpenOptionsExt {
     ///
     /// ```no_run
     /// # #![feature(rustc_private)]
-    /// use libc;
     /// use std::fs::OpenOptions;
     /// use std::os::unix::fs::OpenOptionsExt;
     ///