Move internal modules to “internal” folder.

.internal/stackHas.js

@@ -0,0 +1,14 @@
+/**
+ * Checks if a stack value for `key` exists.
+ *
+ * @private
+ * @name has
+ * @memberOf Stack
+ * @param {string} key The key of the entry to check.
+ * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+ */
+function stackHas(key) {
+  return this.__data__.has(key);
+}
+
+export default stackHas;