Move internal modules to “internal” folder.

.internal/baseSetData.js

@@ -0,0 +1,17 @@
+import identity from './identity.js';
+import metaMap from './.internal/metaMap.js';
+
+/**
+ * The base implementation of `setData` without support for hot loop shorting.
+ *
+ * @private
+ * @param {Function} func The function to associate metadata with.
+ * @param {*} data The metadata.
+ * @returns {Function} Returns `func`.
+ */
+const baseSetData = !metaMap ? identity : (func, data) => {
+  metaMap.set(func, data);
+  return func;
+};
+
+export default baseSetData;