List modules from command line

We should be able to get the list of installed modules from the plinth command line. I'd suggest the following:

• plinth --list-modules --> Lists all modules.
• plinth --list-modules essential --> Lists essential modules only.
• plinth --list-modules optional --> Lists optional modules only.

This is to support https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=847680.