feature: default-run included in `cargo metadata`

[dzfranklin]

Describe the problem you are trying to solve

I'm writing a subcommand that mirrors cargo, but records things under a time-travelling debugger instead of running them. When someone types cargo rr run I need to get whatever binary cargo would have run.

In #9491 I requested a solution for this specific use case (something like cargo build --default-bin). When looking into replicating the algorithm cargo uses as a workaround I learned cargo doesn't expose the field default_run in cargo metadata.

Describe the solution you'd like

The default_run field under [package] in Cargo.toml is included in cargo metadata

Notes

Based on a cursory look I think this would just require adding a field to SerializedPackage and changing Package::serialized to propagate the field value from Manifest. I'd be happy to file a PR for this.

[joshtriplett]

We discussed this in today's @rust-lang/cargo meeting, and we'd be entirely in favor.

[0xPoe]

@danielzfranklin @ehuss I would like to try to add it.

[0xPoe]

@rustbot claim