\Org_Heigl\Holidaychecker\IteratorItem\Easter relies on functions provided by ext-calendar.

DOMElement (used in \Org_Heigl\Holidaychecker\Factory\SwapDecoratorFactory) is provided by ext-dom.

Instead of requiring ext-calendar it is also possible to require roukmoute/polyfill-calendar. This is nice, because it will provide the functions needed for the Easter calculation when ext-calendar is missing and will do nothing if not.