Improve integration of the http server introduced by the http-registry feature.
Now the same HTTP server is used for serving downloads, the index, and
the API.
This makes it easier to write tests that deal with authentication and
http registries.
Benefits:
- A TOML 1.0 compliant parser
- Unblock future work
- Have `cargo init` add the current crate to the workspace, rather
than error
- #5586: Upstream `cargo-add`
