Add a JSON output mode to cargo package.

[LawnGnome]

In multi-crate workspaces, it can be ambiguous where files are sourced from when building packages. This extends the existing --list command line flag to optionally emit JSON when called as -l=json or --list=json. Existing behaviour is unaffected.

The JSON format is slightly different to that proposed in #11666: it exposes the distinction within cargo::ops::cargo_package itself around whether an archive file is generated or sourced from the disk. A new crate created with cargo new --lib foo generates this output:

{
  ".cargo_vcs_info.json": {
    "Generated": {
      "VcsInfo": {
        "git": {
          "sha1": "af0bf5917934ec4684cd23763f90c16242d7c5ce"
        },
        "path_in_vcs": ""
      }
    }
  },
  ".gitignore": {
    "OnDisk": "/tmp/foo/.gitignore"
  },
  "Cargo.toml": {
    "Generated": "Manifest"
  },
  "Cargo.toml.orig": {
    "OnDisk": "/tmp/foo/Cargo.toml"
  },
  "src/lib.rs": {
    "OnDisk": "/tmp/foo/src/lib.rs"
  }
}

I'm open minded as to whether the VcsInfo should actually be included, but it feels like there's little harm in doing so.

Fixes #11666, and is at least extremely relevant to #13953.

[rustbot]

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[epage]

FYI I'm moving this to Draft as this has not been accepted or needs-mentor yet.

We generally reserve PRs for discussing the implementation and leaving problem, requirements, and solution discussions to the issues. I'd recommend furthering the discussion on the Issue.

[bors]

☔ The latest upstream changes (presumably #13947) made this pull request unmergeable. Please resolve the merge conflicts.