Missed elm/json project on indirect dependencies.

Hi @avh4 !!

I started one poc project and happend a problem.

My steps

1. elm init
2. elm install elm-test
3. create a simple test
4. Run and test was ok.
5. elm-test install avh4/elm-program-test
6. Run and got a error about invalidt elm.json.
[Repo with bug](https://github.com/Bernardoow/report/tree/with_bug)

So I compared with another package my that everything works, so I started install some packages to find the problem.

I installed elm/http and it continued with the problem. So I installed a elm/json and the tests go back works.
[Repo without bug](https://github.com/Bernardoow/report/tree/without_bug)

Start by comparing the linked with_bug and without_bug repositories, following the reported elm init, elm install, and elm-test install steps. Inspect the generated elm.json files and dependency resolution around elm-test, avh4/elm-program-test, elm/http, and elm/json; done means the indirect dependency is included without requiring a manual elm/json installation.

Written by the indexing model from the issue text.