Include registry names in cargo metadata output

[sunshowers]

Describe the problem you are trying to solve
For some automated tooling built on top of guppy, I'd like to be able to reconstruct Cargo.toml dependency lines from cargo metadata output. This is possible to do for crates.io, path and Git dependencies, but doesn't appear to be possible to do for non-default registries.

Specifically, the bit that's missing is the name of the registry. The output format has a "registry" field in the dependencies section, but that contains the URL and not the name. The URL cannot be specified in Cargo.toml's registry field -- it must be a name.

Describe the solution you'd like
There's a few ways to do this, but I think the cleanest way would be to add a new top-level section to cargo metadata output, say registries, which has:

• the default registry along with its URL (typically crates.io but could be something else)
• any other registries in use along with their URLs.
• possibly other details about each registry?

[sunshowers]

I guess another way would be to include the registry name in each URL (e.g. "registry+https://github.com/crates.io/index?name=crates-io", though that's a bit more of a breaking change.)

edit: I think this might be the correct way to do it, actually -- not sure if you can specify multiple registry names with the same index, etc.