Fix command-line when we don't have a preferred KSP install.

This is work related to #188, and makes sure a user can use the
`ckan ksp ...` commands to manipulate which KSP version they're
working with, without the command-line client trying to find
a KSP instance first.

This also involves a bit of restructing internally. In particular,
the ksp command (add, remove, rename, forget, list, etc) is
now broken out into its own file.