Move internal modules to “internal” folder.

.internal/isFlattenable.js

@@ -0,0 +1,15 @@
+import isArguments from './isArguments.js';
+
+/**
+ * Checks if `value` is a flattenable `arguments` object or array.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is flattenable, else `false`.
+ */
+function isFlattenable(value) {
+  return Array.isArray(value) || isArguments(value) ||
+    !!(Symbol.isConcatSpreadable && value && value[spreadableSymbol]);
+}
+
+export default isFlattenable;