Move internal modules to “internal” folder.

.internal/stackClear.js

@@ -0,0 +1,15 @@
+import ListCache from './.internal/ListCache.js';
+
+/**
+ * Removes all key-value entries from the stack.
+ *
+ * @private
+ * @name clear
+ * @memberOf Stack
+ */
+function stackClear() {
+  this.__data__ = new ListCache;
+  this.size = 0;
+}
+
+export default stackClear;