Move internal modules to “internal” folder.

.internal/isPrototype.js

@@ -0,0 +1,18 @@
+/** Used for built-in method references. */
+const objectProto = Object.prototype;
+
+/**
+ * Checks if `value` is likely a prototype object.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is a prototype, else `false`.
+ */
+function isPrototype(value) {
+  const Ctor = value && value.constructor;
+  const proto = (typeof Ctor == 'function' && Ctor.prototype) || objectProto;
+
+  return value === proto;
+}
+
+export default isPrototype;