plugable: replace API.import_plugins with new API.add_package
    
    Replace API.import_plugins with a new method API.add_package which allows
    loading plugin packages into an API object from a package object.
    
    This makes loading of plugin packages loading consistent with loading of
    plugin modules and classes.
    
    Rename API.modules to API.packages and use package objects where
    implemented to reflect the change.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>