Expose machine_message.rs as a crate so that it's easy for third party tools

[richardwhiuk]

Describe the problem you are trying to solve

Currently https://doc.rust-lang.org/cargo/reference/external-tools.html#json-messages nicely describes the messages which Cargo can output.

However, there's no programmatic definition, so integrations are forced to copy and paste structs out of Cargo's code to parse them.

Describe the solution you'd like

It'd be nice if there was a rust crate which a) Cargo used to define these messages and b) third party developers could pull and use serde::Deserialise to parse them.

[ehuss]

The types are available via the cargo_metadata crate (see the Message type).

[ehuss]

Closing, cargo_metadata is the intended way, particularly if using serde.