Move internal modules to “internal” folder.

.internal/Stack.js

@@ -0,0 +1,29 @@
+import ListCache from './.internal/ListCache.js';
+import stackClear from './.internal/stackClear.js';
+import stackDelete from './.internal/stackDelete.js';
+import stackGet from './.internal/stackGet.js';
+import stackHas from './.internal/stackHas.js';
+import stackSet from './.internal/stackSet.js';
+
+/**
+ * Creates a stack cache object to store key-value pairs.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [entries] The key-value pairs to cache.
+ */
+class Stack {
+  constructor(entries) {
+    const data = this.__data__ = new ListCache(entries);
+    this.size = data.size;
+  }
+}
+
+// Add methods to `Stack`.
+Stack.prototype.clear = stackClear;
+Stack.prototype['delete'] = stackDelete;
+Stack.prototype.get = stackGet;
+Stack.prototype.has = stackHas;
+Stack.prototype.set = stackSet;
+
+export default Stack;