Skip prefetching when `--no-deps` is specified (#2373)
## Summary
When running under `--no-deps`, we don't need to pre-fetch, because
pre-fetching fetches the _distribution_ metadata. But with `--no-deps`,
we only need the package metadata for the top-level requirements. We
never need distribution metadata.
Incidentally, this will fix https://github.com/astral-sh/uv/issues/2300.
## Test Plan
- `cargo test`
- `./target/debug/uv pip install --verbose --no-cache-dir --no-deps
--reinstall ddtrace==2.6.2 debugpy==1.8.1 ecdsa==0.18.0
editorconfig==0.12.4 --verbose` in a Python 3.10 Docker contain
repeatedly.
