Move internal modules to “internal” folder.

.internal/cloneSymbol.js

@@ -0,0 +1,15 @@
+/** Used to convert symbols to primitives and strings. */
+const symbolValueOf = Symbol.prototype.valueOf;
+
+/**
+ * Creates a clone of the `symbol` object.
+ *
+ * @private
+ * @param {Object} symbol The symbol object to clone.
+ * @returns {Object} Returns the cloned symbol object.
+ */
+function cloneSymbol(symbol) {
+  return symbolValueOf ? Object(symbolValueOf.call(symbol)) : {};
+}
+
+export default cloneSymbol;