[Feature Request] Dependency-specific index urls

[johnpyp]

Some packages like pytorch recommend installing their packages through custom index urls, e.g from that page:

# To Install:
pip3 install torch torchvision torchaudio --index-url https://download.pytorch.org/whl/rocm6.0

Though we could use url prioritization (with uv to make it consistent) for this, it would be better in this case to support dependency-scoped index urls, to avoid leaking the extra index url check to every other dependency as well, which introduces an unexpected supply chain scope to all dependencies as well.