Merge #6458

6458: Qualify trait impl created by add_custom_impl assist r=matklad a=Veykril

When we find at least one trait with the same name as the derive accessible from the current module we now generate a qualified path to that trait in the generated impl.
If we don't find any we just do what was done before and emit the trait name in the generated impl.

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>