Maintain a plan.json file for external tools

This is (mostly) hvr's code.

It produces an external representation of the elaborated install plan.
The intention is for it to be used by external tools, editors etc, to
find out relevant details of the project and package configuration and
environment. It is updated whenever the elaborated install plan
changes, so it always reflects the current state.

Currently it only includes a modest subset of the details from the
elaborated install plan, but the goal is to include all relevant details
in a mostly-stable JSON format.

Currently it's based on the elaborated install plan prior to improving
the plan with packages from the store, but ultimately it will need to
include information from both before and after the 'improvement' phase.