In particular it will not reflect the status of the build, ie it will
list all the source packages and their configuration but not say if
they're already installed in the store.

There's a slightly vauge principle at work here, that we should
distinguish between build outputs and build ststus: build outputs
should be (morally) pure functions of the environment, where as build
status is about the degree to which the current cached outputs reflect
the state of the inputs (ie are build outputs up to date or not).

So in this case that means the plan.json is considered an output and
thus it should not depend on the state of the store. We can certainly
provide machine readable status info, but this should be a separate
thing.