Move internal modules to “internal” folder.

.internal/stackGet.js

@@ -0,0 +1,14 @@
+/**
+ * Gets the stack value for `key`.
+ *
+ * @private
+ * @name get
+ * @memberOf Stack
+ * @param {string} key The key of the value to get.
+ * @returns {*} Returns the entry value.
+ */
+function stackGet(key) {
+  return this.__data__.get(key);
+}
+
+export default stackGet;