tests: move GroupCommand tests to a more proper place.
GroupsCommand: make the way comps are searched more robust. (RhBug:1051869)

Also fixes some related problems introduced with the recent GroupCommand
changes.
refactor: commands.group: _split_extcmd is a static method.
refactor: put DistoSyncCommand into its own file
show plugin commands in separate block
api: add new public Command.usage & Command.summary API
cleanup: make Commands use the new api