Move internal modules to “internal” folder.

.internal/arrayFilter.js

@@ -0,0 +1,24 @@
+/**
+ * A specialized version of `filter` for arrays.
+ *
+ * @private
+ * @param {Array} [array] The array to iterate over.
+ * @param {Function} predicate The function invoked per iteration.
+ * @returns {Array} Returns the new filtered array.
+ */
+function arrayFilter(array, predicate) {
+  let index = -1;
+  let resIndex = 0;
+  const length = array == null ? 0 : array.length;
+  const result = [];
+
+  while (++index < length) {
+    const value = array[index];
+    if (predicate(value, index, array)) {
+      result[resIndex++] = value;
+    }
+  }
+  return result;
+}
+
+export default arrayFilter;