Support for same-prefix uses for merge-imports assist

Follow-up from https://github.com/rust-analyzer/rust-analyzer/pull/3745#issue-394855923

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(?)

Start by reading the linked PR #3745 and locating the merge-imports assist and its existing tests. Check whether the assist owns same-prefix merging and investigate the wildcard-use edge case described here. Done means the assist transforms use foo::Bar; plus use foo::*; into only use foo::*;, with coverage for the relevant behavior.