Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Description of change

Inside #607 (comment) I made a hack to allow me to iterate over the different types of resources.

This PR introduces the same pattern to resources as exists already for outputs.

I spent an afternoon trying to come up with a way to cover this in a unit test that would start to fail if a new resource type was ever added, but didn't come up with something. I tried iterating over the exported structs inside of the resources package, but failed to find a way of checking whether they implemented the Resource interface; golang doesn't maintain a type registry, so this was some pretty gnarly reflection.

Having made that attempt, I actually now think it would be simpler to do something like:

var resourceTypes = map[string]Resource{
  "addr": Addr{},
  "command": Command{},
  ...
  "user": User{},
}

func Resources() map[string]Resource {
  return resourceTypes
}

and similarly for Outputers (but where the Outputer interface grows a FormatOptions() []string that returns a constant set.

The cons are that to add a new one, you have to remember add an entry to the map. I think this is no worse than the current setup, where you must remember to make the init that calls RegisterOutputer.

The pros, though:

• no longer need the init entry per file (3 lines of code * 16)
• no need for the RegisterResource or RegisterOutputer funcs, or mutexes, because the maps are already statically defined and externally, they're read-only via the getter func (unless I've misunderstood how by-value/by-ref works, of course, it wouldn't be the first time)
• no need for the mutexes inside the of the Outputers() or FormatOptions()
• since Outputer.FormatOptions() would now exist, IsValidFormatOption() becomes a map lookup

What do you think? I'd be happy to make a separate PR to change to this approach for Outputer, so you can see it, and if you like that more, that could merge and then I could adapt this one.