Preserve lowered extra build dependencies (#15038)
## Summary
I should've noticed this during review -- my bad -- but it looks like
after lowering, we're converting back to `uv_pep508::Requirement`. This
is mostly okay, but it's lossy for some lowerings. For example, we lose
index pinning. With this PR, we now preserve the lowered types
(`Requirement`).
Closes https://github.com/astral-sh/uv/issues/15037.
