Support for overriding dl and api in config.json of registry index

[chenyuanrun]

See #6219 .

dl and api can be overrided in cargo config file like:

[source.crates-io]
replace-with = "dl-test"

[source.dl-test]
registry = "https://mirrors.tuna.tsinghua.edu.cn/git/crates.io-index.git"
dl = "https://crates-io.proxy.ustclug.org/api/v1/crates"

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[bors]

☔ The latest upstream changes (presumably #10907) made this pull request unmergeable. Please resolve the merge conflicts.

[weihanglo]

Sorry for the extremely long wait 😅.

I am not quite sure the conclusion of #6219. Just what to clarify the intention of this change again:

• Why the feature should be a built-in feature of Cargo.
• What the current solution/workaround looks like without this feature, and how painful it is.
• Does the config value http.proxy cover the need of this being built-in?

Also, I am not quite sure how it plays well with alternative registry authentication and the incoming asymmetric token authentication. How should overridden URLs interact with those authenticated endpoints? Should they just ignore, bypass or reuse the same auth token (which might be risky)? Would recommend we get back to #6219 for further discussions.

Hope I didn't get the purpose of this PR wrong. Thank you.

[weihanglo]

Going to close this, as there are some unresolved design issues pending to be sorted out. Also, registry interaction internals has been changed a bit since then. I'd suggest having discussions in #6219 or on Zulip first. Thank you for putting effort on this :)