Move internal modules to “internal” folder.

.internal/stackSet.js

@@ -0,0 +1,33 @@
+import ListCache from './.internal/ListCache.js';
+import MapCache from './.internal/MapCache.js';
+
+/** Used as the size to enable large array optimizations. */
+const LARGE_ARRAY_SIZE = 200;
+
+/**
+ * Sets the stack `key` to `value`.
+ *
+ * @private
+ * @name set
+ * @memberOf Stack
+ * @param {string} key The key of the value to set.
+ * @param {*} value The value to set.
+ * @returns {Object} Returns the stack cache instance.
+ */
+function stackSet(key, value) {
+  let data = this.__data__;
+  if (data instanceof ListCache) {
+    const pairs = data.__data__;
+    if (!Map || (pairs.length < LARGE_ARRAY_SIZE - 1)) {
+      pairs.push([key, value]);
+      this.size = ++data.size;
+      return this;
+    }
+    data = this.__data__ = new MapCache(pairs);
+  }
+  data.set(key, value);
+  this.size = data.size;
+  return this;
+}
+
+export default stackSet;