Move internal modules to “internal” folder.

.internal/baseIsSet.js

@@ -0,0 +1,18 @@
+import getTag from './.internal/getTag.js';
+import isObjectLike from './isObjectLike.js';
+
+/** `Object#toString` result references. */
+const setTag = '[object Set]';
+
+/**
+ * The base implementation of `isSet` without Node.js optimizations.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is a set, else `false`.
+ */
+function baseIsSet(value) {
+  return isObjectLike(value) && getTag(value) == setTag;
+}
+
+export default baseIsSet;