Include info needed to build backpack-using packages in plan.json

[shlevy]

Please read Github PR Conventions and then fill in one of these two templates.

---

This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[shlevy]

This is a draft while I confirm this info is sufficient and then I’ll do all the code formatting and changelog etc.

[shlevy]

This did turn out to be the info I needed, but it also turns out that haskell.nix vendors its own version of ProjectPlanOutput.hs. I’m happy to clean this up if the maintainers want it, but it’s no longer directly valuable to me.