Order item Views module integration should verify purchasable entity type definition still exists

Spin off from #2913523: Order item referencing addon entity breaks order item Views integration. Minor as its an edge case.

OrderItemViewsData builds a relationship dynamically from the order item to the purchasable entity type based on the order item type's purchasable entity.

    // Provide a relationship for each entity type found.
    foreach ($entity_type_ids as $entity_type_id) {
      /** @var \Drupal\Core\Entity\EntityTypeInterface $entity_type */
      $entity_type = $this->entityManager->getDefinition($entity_type_id);

This line of code breaks, however, if that entity has disappeared and Drupal didn't remove the order item type configuration.

We need to add a `hasDefinition` check before assuming the definition exists.