Fix registry download URL template expansion

[syphar]

Fix IndexConfig download URL expansion to match Cargo’s registry-index format.

• Support the {sha256-checksum} download URL marker.
• Add IndexConfig::download_url_with_checksum.
• Make Version::download_url supply the checksum from its index entry.
• Preserve case for {prefix} and lowercase only {lowerprefix}.
• Keep sparse-index and local cache paths lowercase, as required by Cargo.
• Add coverage for download URL templates, mixed-case prefixes, local paths, and sparse crate URLs.

This does not change normal crates.io download URLs or sparse paths. It fixes compatibility with registries that use checksum-based or case-sensitive download URL templates.

[syphar]

I stumbled onto this when working on the other fixes.

For docs.rs I don't need the fix, but I thought there is value in it anyways.

[Byron]

Thanks a lot, it's much appreciated!