Add isolated container and dhii modularity deps

lib/packages/Dhii/Collection/CountableListInterface.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WooCommerce\PayPalCommerce\Vendor\Dhii\Collection;
+
+use Traversable;
+use Countable;
+
+/**
+ * A list that can be counted.
+ *
+ * @since 0.2
+ */
+interface CountableListInterface extends
+    /* @since 0.2 */
+    Traversable,
+    /* @since 0.2 */
+    Countable
+{
+}