Fix spans in unused import lint for nested groups

[pietroalbini]

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

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

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[petrochenkov]

@bors r+

[bors]

📌 Commit 0847ac0 has been approved by petrochenkov