Add an API to generate registry index data

See RFC 2141 for more details/context.

Currently, the knowledge of what goes in the registry index in what format lives in crates.io:

• here,
• which serializes an instance of this struct,
• which comes from here,
• most of which comes from cargo anyway, around here,
• which deserializes an instance of this struct
• which has a corresponding struct in cargo
• that gets created here

In order to support minimal registries, we'd like cargo to be able to generate the JSON information that should go into a registry for the current crate.

Cargo does already understand how to read these entries! We need to teach it how to create them as well :)

So as I understand it, this would involve:

• Derive Serialize for the RegistryPackage struct
• Create a new command, feel free to suggest another name, the first thought in my head is generate-index-metadata, that should:
  • Create an instance of RegistryPackage for the current crate, reusing code that sends relevant information to crates.io when appropriate
  • Serialize that struct to stdout

Start with RFC 2141 and the RegistryPackage struct in src/cargo/sources/registry/mod.rs, then read the registry operation that sends package information to crates.io in src/cargo/ops/registry.rs. Trace how Cargo reads registry entries before deciding how the new command should construct and serialize the current crate's metadata. Done means a command generates the registry JSON and writes it to stdout.