Fix error in `indico setup list-plugins`
Fix error in `indico setup list-plugins`

Loading the plugins results in imports running, and if some models
haven't been imported this may cause errors in some cases (e.g. in case
of marshmallow ModelSchemas that reference a model which in turn
references another model that has not been imported yet).

indico/indico,pferreir/indico,pferreir/indico,mvidalgarcia/indico,OmeGak/indico,mvidalgarcia/indico,indico/indico,mvidalgarcia/indico,mic4ael/indico,OmeGak/indico,ThiefMaster/indico,mvidalgarcia/indico,DirkHoffmann/indico,DirkHoffmann/indico,indico/indico,mic4ael/indico,DirkHoffmann/indico,OmeGak/indico,DirkHoffmann/indico,mic4ael/indico,pferreir/indico,OmeGak/indico,mic4ael/indico,ThiefMaster/indico,ThiefMaster/indico,ThiefMaster/indico,indico/indico,pferreir/indico