Query latest version of crates from local registry index rather than HTTP API

[DCjanus]

Obviously, querying versions via the HTTP API is slow, maybe we could query from local registry index.

ref: #292 (comment)

[DCjanus]

The points of this feature in my mind for now:

• Cargo configuration: https://doc.rust-lang.org/cargo/reference/config.html#hierarchical-structure
• Source replacement: https://doc.rust-lang.org/cargo/reference/source-replacement.html#source-replacement
• Custom api format: https://doc.rust-lang.org/cargo/reference/registries.html#index-format

[DCjanus]

I'd like to work for this, but in the next half month I may not have enough spare time. If someone would like to help me, that will be great.

And I don't know how to read files in bare git repo.

[DCjanus]

Blocked by rust-lang/cargo#7075

Implemented by hand