Move internal modules to “internal” folder.

.internal/cloneSet.js

@@ -0,0 +1,22 @@
+import addSetEntry from './.internal/addSetEntry.js';
+import arrayReduce from './.internal/arrayReduce.js';
+import setToArray from './.internal/setToArray.js';
+
+/** Used to compose bitmasks for cloning. */
+const CLONE_DEEP_FLAG = 1;
+
+/**
+ * Creates a clone of `set`.
+ *
+ * @private
+ * @param {Object} set The set to clone.
+ * @param {Function} cloneFunc The function to clone values.
+ * @param {boolean} [isDeep] Specify a deep clone.
+ * @returns {Object} Returns the cloned set.
+ */
+function cloneSet(set, isDeep, cloneFunc) {
+  const array = isDeep ? cloneFunc(setToArray(set), CLONE_DEEP_FLAG) : setToArray(set);
+  return arrayReduce(array, addSetEntry, new set.constructor);
+}
+
+export default cloneSet;