POC: Sparse protocol support

[inflation]

Quick and dirty way to support sparse protocol and set it by default.

The proper way is to support registry selection and protocol type detection.

[obi1kenobi]

Very interesting, thanks for putting this POC together!

Regarding the protocol type detection, is there a good way for us to use crates_index to do that? Or is that something that crates_index will have to build and expose to us?

Also, by using sparse protocol by default, are we likely to expose Windows users to this bug? frewsxcv/rust-crates-index#126

[inflation]

I guess it's one of the internal things that cargo should "cratelize" for reusing. It need to parse through the hierarchy of config.toml to figure out the index URL of the specified registry. Then we can determine if it's sparse or not by checking the sparse+ prefix.

Windows should be fine since reqwest handles it anyway.

[obi1kenobi]

Since this PR, the crates_index became unmaintained but then thanks to the author of tame-index, we switched to that crate and gained sparse index support in #506.

The next release of cargo-semver-checks will have sparse index support 🎉 Thanks for the help and for your patience on this!