[Vuejs]-How to use an external module inside a Vue SFC?

You need to register it globally using Vue.component('v-calendar',Calendar ) or in case of class based SFC you are importing Calendar a named import, then it should be used as <calendar></calendar>
@Component({ components: { setupCalendar, 'v-calendar':Calendar } }) can also be used in class based SFCs to provide a custom tag name to some imported component.