Move internal modules to “internal” folder.

.internal/nativeKeysIn.js

@@ -0,0 +1,20 @@
+/**
+ * This function is like
+ * [`Object.keys`](http://ecma-international.org/ecma-262/7.0/#sec-object.keys)
+ * except that it includes inherited enumerable properties.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Array} Returns the array of property names.
+ */
+function nativeKeysIn(object) {
+  const result = [];
+  if (object != null) {
+    for (const key in Object(object)) {
+      result.push(key);
+    }
+  }
+  return result;
+}
+
+export default nativeKeysIn;