add generic sparse registry support, use it for crates.io

[syphar]

• adds support for sparse registries. For now, only used in integration tests in docs.rs, but IMO the support generally is good to have
• the crates_io support now just uses the sparse registry support
• renames "alternate registry" with "git registry" support, what it (now) actually is. Tries to keep backwards compatibility with some aliases / re-exports, not sure if that's necessary or we just do the breaking change.
• we previously didn't support all placeholders for download urls, I didn't fix that in this PR. Doing so would also mean fetching the checksum from the (git/sparse) index, so we can include it in the download url. See also Fix registry download URL template expansion frewsxcv/rust-crates-index#201 and the cargo registry docs. Not sure if that's worth the time / effort, or if we even should start using the crates-index lib for some of the features in here.

[syphar]

r? @GuillaumeGomez

[GuillaumeGomez]

Thanks!