Include public/private dependency status in `cargo metadata`

[eopb]

Problem

related to rust-lang/rust#44663

Information about whether or not a dependency is public is not included in the output of cargo metadata.

This could be useful for custom subcommands. cargo override, would find this useful for eopb/cargo-override#121

Proposed Solution

When -Zpublic-dependency is enabled, include a boolean flag in cargo metadatas output at packages[].dependencies[].public.
If -Zpublic-dependency is not enabled, this flag should not appear in cargo metadata output.

Notes

If this feature request is accepted, I'm happy to claim and take a crack at working on it

[epage]

This seems like basic enough of metadata for us to include.

[eopb]

@rustbot claim