uv-resolver: deduplicate resolution markers (#9780)
Since we don't (currently) include conflict markers with our
`resolution-markers` in the lock file, it's possible that we end up
with duplicate markers. This happens when the resolver creates more
than one fork with the same PEP 508 markers but different conflict
markers, _and_ where those PEP 508 markers don't simplify to "always
true" after accounting for `requires-python`.
This change should be a strict improvement on the status quo. We aren't
removing any information. It is possible that we should be writing
conflict markers here (like we do for dependency edges), but I haven't
been able to come up with a case or think through a scenario where they
are necessary.
Fixes #9296
