Output harness in cargo metadata

[jordanisaacs]

What does this PR try to resolve?

Adds harness to output of cargo metadata. Closes #11846 . Useful for third party rust build tools that want to use Cargo.toml metadata.

How should we test and review this PR?

Updated the metadata tests with the new harness output.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

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[ehuss]

Thanks! It looks like there is a test failure, you'll need to run tests with nightly to check benchmarks.

Can you also update the documentation at https://github.com/rust-lang/cargo/blob/master/src/doc/man/cargo-metadata.md, and regenerate the man pages (documented at https://github.com/rust-lang/cargo/tree/master/src/doc#building-the-man-pages)? Also https://github.com/rust-lang/cargo/blob/master/src/doc/src/reference/external-tools.md looks to also need updating.

[weihanglo]

Ping @jordanisaacs. Just checking in to see if you are still interested in working on this. As Eric mentioned in #11847 (comment), there are some doc updates needing to be done, and we're ready to go!

[ehuss]

I'm going to close due to inactivity. If you are interested in continuing on this, feel free to reopen (just beware that due to a quirk in GitHub, if you push to your branch before reopening that you'll need to open a new PR instead).
Thanks for your contribution.