Avoid treating localhost URLs as local file paths (#3132)
## Summary
Closes https://github.com/astral-sh/uv/issues/3128.
## Test Plan
- `python -m http.server`
- `cargo run pip install
"http://localhost:8000/werkzeug-3.0.2-py3-none-any.whl"`
- `cargo run pip install
"http://localhost:8000/werkzeug-3.0.2-py3-none-any.whl"`
