handle {self} when removing unused imports

[harrysarson]

Fixes #17139

On master

mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner::{self, X}$0;

    fn f() {
        let y = inner::Y();
    }
}

becomes

mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner:self;

    fn f() {
        let y = inner::Y();
    }
}

with this fix it instead becomes

```rs
mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner;

    fn f() {
        let y = inner::Y();
    }
}

[Veykril]

That transform is generally not valid, use foo::bar::{self} imports only the type namespace, where as use foo::bar; imports value, type and macro namespace. So transforming like that can actually cause name collisions.

Now there is still a bug here in that we produce malformed syntax but we need to keep the braces.

[Veykril]

This can be a closure

[harrysarson]

I looked to make this into a closure, and in the process decided to check the number of use_tree children as part of the same check (so now the closure is called has_single_subtree_that_is_not_self)

[harrysarson]

That transform is generally not valid, use foo::bar::{self} imports only the type namespace, where as use foo::bar; imports value, type and macro namespace. So transforming like that can actually cause name collisions.

Now there is still a bug here in that we produce malformed syntax but we need to keep the braces.

Yep. I have removed the second commit, the first commit (which fixes remove_unnecessary_braces) I think is what we need to fix the malformed syntax and keep the braces.

[harrysarson]

That transform is generally not valid,

The normalize import IDE assist (https://github.com/rust-lang/rust-analyzer/blob/master/crates/ide-assists/src/handlers/normalize_import.rs) does transform {self}. I.e. the following unit test would pass if I added it to normalize_import.rs

    #[test]
    fn test_collapses_self() {
        check_assist_variations!("std::fmt::{self}", "std::fmt");
    }

Is that a bug?