Package spec in build plan and metadata?

[Hoverbear]

Describe the problem you are trying to solve

I'd like to selectively prebuild some or all of the dependencies of my crate.

I can do this with cargo build --package log which is super awesome.

If I have two version of log, I need to specify which one with a package spec. Cargo gives me the ability to do cargo metadata and cargo build --build-plan, it also gives me the option to get an ID with cargo pkgid (https://doc.rust-lang.org/cargo/reference/pkgid-spec.html)

Unfortunately, I can't make them play together. metadata and --build-plan don't provide package specs, so I end up forced to do manual parsing if I want to hope to make all these tools interact.

Describe the solution you'd like

Include a package spec in the output of cargo metadata and/or cargo build --build-plan

Notes
Related to #2644 since that was essentially what I was trying to do.

[epage]

If #7289 isn't a duplicate, then its closely related.

EDIT: #7289 is about adding PackageId to another struct, not about allowing integration with PackageIdSpec

[epage]

Hmm, it appears package specs are ambiguous and we'd need to solve that first.

While working to improve the design of package id specs, we should include resolving #7725.