Move internal modules to “internal” folder.

.internal/baseUpdate.js

@@ -0,0 +1,18 @@
+import baseGet from './.internal/baseGet.js';
+import baseSet from './.internal/baseSet.js';
+
+/**
+ * The base implementation of `update`.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {Array|string} path The path of the property to update.
+ * @param {Function} updater The function to produce the updated value.
+ * @param {Function} [customizer] The function to customize path creation.
+ * @returns {Object} Returns `object`.
+ */
+function baseUpdate(object, path, updater, customizer) {
+  return baseSet(object, path, updater(baseGet(object, path)), customizer);
+}
+
+export default baseUpdate;