uv-resolver: fix perf regression
We significantly regressed performance in some cases because we were
cloning the resolver state one more time than we needed to. That doesn't
sound like a lot, but in the case where there are no forks, it implies
we were cloning the state for every `get_dependencies` called when we
shouldn't have been cloning it at all.
Avoiding the clone results in somewhat tortured code. This can probably
be refactored by moving bits out to a helper routine, but that also
seemed non-trivial. So we let this suffice for now.
