Move internal modules to “internal” folder.

.internal/createInverter.js

@@ -0,0 +1,14 @@
+import baseInverter from './.internal/baseInverter.js';
+
+/**
+ * Creates a function like `invertBy`.
+ *
+ * @private
+ * @param {Function} setter The function to set accumulator values.
+ * @returns {Function} Returns the new inverter function.
+ */
+function createInverter(setter) {
+  return (object, iteratee) => baseInverter(object, setter, iteratee, {});
+}
+
+export default createInverter;