Support for same-prefix uses for merge-imports assist

[pszpetkowski]

Follow-up from #3745 (comment)

I believe it might be a low-hanging fruit to add support for merging same-prefix uses as pointed out in linked PR:

use foo::Bar;
use foo::*;

transformed into after the assist call:

use foo::*;

There are few concerns I'd like to point out here:

1. I've not investigated the effort which would have to be done to implement it.
2. I'm not entirely sure if it's a responsibility of this assist
3. There might be an edge case where separate use along with wildcard use makes something work(?)