handle {self} when removing unused imports
Fixes#17139
On master
```rs
mod inner {
pub struct X();
pub struct Y();
}
mod z {
use super::inner::{self, X}$0;
fn f() {
let y = inner::Y();
}
}
```
becomes
```rs
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();
}
}
```
e3e22c67e0