Snapcraft help uses many imports

[Daniel Holbach]

We'd like to be able to run ./snapcraft/scripts/generate_reference.py on the developer site without having to install all dependencies of snapcraft and having to watch closely how the dependency chain changes. It'd be good if the following commands used less imports:

 - help
 - list-plugins

[Leo Arias (elopio)]

Makes sense.
But I think this will have to wait until we move our help out of docopt.