Avoid forking for identical markers (#10490)
## Summary
If you have a dependency with a marker, and you add a constraint, it
causes us to _always_ fork, because we represent the constraint as a
second dependency with the marker repeated (and, therefore, we have two
requirements of the same name, both with markers). I don't think we
should fork here -- and in the end it's leading to this undesirable
resolution: #10481.
I tried to change constraints such that we just _reuse_ and augment the
initial requirement, but that has a fairly negative effect on error
messages: #10489. So this fix seems a bit better to me.
Closes https://github.com/astral-sh/uv/issues/10481.
