Allow unused modules to be disabled

The farmOS distribution declares all of the modules it includes as dependencies - this prevents them from being disabled after install.

So if you have a farm that does not raise livestock, you're stuck with the livestock module enabled, and you can't disable it without hacking the dependency out of farmOS core.

The reason they are included as dependencies is to auto-install them during farmOS installation. Mainly for user experience reasons: it's easier to turn everything on at first than to explain how to turn things on after the fact.

One way to fix this would be to simply install them via hook_install(), rather than declaring them as dependencies. It would be the same end-result, but would also allow them to be disabled after the fact.

A more complete solution is this: #2651400: Configurable installation