fix hook groups definition to have an ID - switch to using JSON in the file?

The form submission has data about hook groups which is pretty useless as all there is to go on is the human-readable label rather than any kind of machine key.

But in order to have this, the definition file needs to use an ID.

I'd like to switch to using something like JSON rather than a custom made-up format, but I'm concerned about the lack of a drupal_json_decode() on versions lower than 7. So it may be we're stuck with this format for the time being and just have to expand it a little.