Remove Commerce Product dependency from Commerce Cart and Commerce Checkout

The changes look weird to me, besides, I don't want to risk introducing regressions at this point... Why are we now conditionally swapping the add to cart form only if the product module is enabled?

+  if (\Drupal::moduleHandler()->moduleExists('commerce_product')) {
+    $entity_types['commerce_order_item']->setFormClass('add_to_cart', '\Drupal\commerce_cart\Form\AddToCartForm');
+  }

So no add to cart if the product module isn't enabled? That is the change that looks weird to me.

I guess this is due to this logic:

if ($selected_variation && $order_item->getPurchasedEntityId() != $selected_variation) {
        /** @var \Drupal\commerce_product\Entity\ProductVariationInterface $selected_variation */
        $selected_variation = $this->entityTypeManager->getStorage('commerce_product_variation')->load($selected_variation);
        if ($selected_variation->getOrderItemTypeId() !== $order_item->bundle()) {
          /** @var \Drupal\commerce_order\OrderItemStorageInterface $order_item_storage */
          $order_item_storage = $this->entityTypeManager->getStorage('commerce_order_item');
          $order_item = $order_item_storage->createFromPurchasableEntity($selected_variation);
          $this->prepareOrderItem($order_item);
          $this->setEntity($order_item);
          $form_display = EntityFormDisplay::collectRenderDisplay($order_item, $this->operation);
          $this->setFormDisplay($form_display, $form_state);
          $form = $original_form;
          $form = parent::buildForm($form, $form_state);
        }
      }

But it probably wouldn't trigger if:

$selected_variation = $form_state->get('selected_variation');

is NULL.

I'm not sure why you're setting this task to "needs review"... As far as I can see, the patch submitted has 42 failing tests in case you haven't noticed.

@facine: Thank you! For some reason when applying the patch, the config files weren't properly moved to the optional directory.
Had to spend time fixing other test failures to make sure this didn't introduce a regression, but tests are green! Good job :).