Enable unnamed requirements for direct URLs (#2569)
## Summary
This PR enables `uv pip install` to accept unnamed requirements, as long
as the requirement ends with the wheel or source distribution archive
name. For example: `cargo run pip install
~/Downloads/anyio-4.3.0.tar.gz`.
In subsequent PRs, I'll expand the scope of supported archives and
patterns.
Part of: https://github.com/astral-sh/uv/issues/313.
