Group unused import warnings per path list

Group unused import warnings per path list

Given a file

```rust
use std::collections::{BinaryHeap, BTreeMap, BTreeSet};

fn main() {}
```

Show a single warning, instead of three for each unused import:

```nocode
warning: unused imports, #[warn(unused_imports)] on by default
 --> foo.rs:1:24
  |
1 | use std::collections::{BinaryHeap, BTreeMap, BTreeSet};
  |                        ^^^^^^^^^^  ^^^^^^^^  ^^^^^^^^
```

Include support for lints pointing at `MultilineSpan`s, instead of just
`Span`s.