output structured json in index.json

The creation of structured json in python is a little awkward, since it's essentially alternating nested lists/dicts/list/dicts... and so on. I'm quite sure there's a way to improve on what I have here for that.

I went ahead and created a schema for index.json based on what I put in #107 (closed). I'm not entirely sure that this is sufficient for implementing the desktop installer (I'm like, 95% sure), so it may need to be tweaked later on. With the schema, tests can now validate the written index.json against it, and it might help anyone else who wants to use index.json.

fixes #107 (closed)