Use redirected URL as base for relative paths (#1816)
## Summary
If you review the setup in https://github.com/astral-sh/uv/issues/1747,
when we fetch `http://localhost:8000/simple/wheel/`, it gets redirected
to `http://localhost:8000/index/wheel/`. So any relative paths returned
need to be resolved relative to `http://localhost:8000/index/wheel/`.
Closes https://github.com/astral-sh/uv/issues/1747.
## Test Plan
- Install `proxpi gunicorn pypiserver`
- `gunicorn proxpi.server:app --bind 0.0.0.0:8000`
- `pypi-server run -p 8080 ~/packages --fallback-url
"http://localhost:8000/index" --verbose`
- `echo "wheel" | cargo run pip compile - --index-url
http://localhost:8080/simple --verbose --no-cache`
