Avoid enforcing extra-only constraints (#4570)
## Summary
In the dependency refactor (https://github.com/astral-sh/uv/pull/4430),
the logic for requirements and constraints was combined. Specifically,
we were applying constraints _before_ filtering on markers and extras,
and then applying that same filtering to the constraints. As a result,
constraints that should only be activated when an extra is enabled were
being enabled unconditionally.
Closes https://github.com/astral-sh/uv/issues/4569.
req.txt