Move internal modules to “internal” folder.

.internal/SetCache.js

@@ -0,0 +1,29 @@
+import MapCache from './.internal/MapCache.js';
+import setCacheAdd from './.internal/setCacheAdd.js';
+import setCacheHas from './.internal/setCacheHas.js';
+
+/**
+ *
+ * Creates an array cache object to store unique values.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [values] The values to cache.
+ */
+class SetCache {
+  constructor(values) {
+    let index = -1;
+    const length = values == null ? 0 : values.length;
+
+    this.__data__ = new MapCache;
+    while (++index < length) {
+      this.add(values[index]);
+    }
+  }
+}
+
+// Add methods to `SetCache`.
+SetCache.prototype.add = SetCache.prototype.push = setCacheAdd;
+SetCache.prototype.has = setCacheHas;
+
+export default SetCache;