Fix spans in unused import lint for nested groups
This fixes an inconsistency for empty nested groups, and adds a test for all the possible cases of the lint.
```
warning: unused imports: `*`, `Foo`, `baz::{}`, `foobar::*`
--> test.rs:16:11
|
16 | use foo::{Foo, bar::{baz::{}, foobar::*}, *};
| ^^^ ^^^^^^^ ^^^^^^^^^ ^
|
= note: #[warn(unused_imports)] on by default
warning: unused import: `*`
--> test.rs:17:24
|
17 | use foo::bar::baz::{*, *};
| ^
warning: unused import: `use foo::{};`
--> test.rs:18:1
|
18 | use foo::{};
| ^^^^^^^^^^^^
```
cc #44494