Respect visitation order for proxy packages (#10833)
## Summary
This PR reverts https://github.com/astral-sh/uv/pull/10441 and applies a
different fix for https://github.com/astral-sh/uv/issues/10425.
In #10441, I changed prioritization to visit proxies eagerly. I think
this is actually wrong, since it means we prioritize proxy packages
above _everything_ else. And while a proxy only depends on itself, it
does mean we're selecting a _version_ for the proxy package earlier than
anything else. So, if you look at #10828, we end up choosing a version
for `async-timeout` before we choose a version for `langchain`, despite
the latter being a first-party dependency. (`async-timeout` has a marker
on it, so it has a proxy package, so we solve for it first.)
To fix #10425, we instead need to make sure we visit proxies in the
order we see them. I think the virtual tiebreaker for proxies is
reversed? We want to visit the package we see first, first.
So, in short: this reverts #10441, then corrects the ordering for
visiting proxies.
Closes https://github.com/astral-sh/uv/issues/10828.
