Fetch `--find-links` indexes in parallel (#934)
## Summary
Removes a TODO.
## Test Plan
Tested manually with:
```shell
cargo run -p puffin-cli -- \
pip compile requirements.in -n \
--find-links 'https://download.pytorch.org/whl/torch_stable.html' \
--find-links 'https://storage.googleapis.com/jax-releases/jax_cuda_releases.html' \
--verbose
```
And inspecting the logs to ensure that the two requests were kicked off
concrurently.
