Move internal modules to “internal” folder.

.internal/mapCacheSet.js

@@ -0,0 +1,22 @@
+import getMapData from './.internal/getMapData.js';
+
+/**
+ * Sets the map `key` to `value`.
+ *
+ * @private
+ * @name set
+ * @memberOf MapCache
+ * @param {string} key The key of the value to set.
+ * @param {*} value The value to set.
+ * @returns {Object} Returns the map cache instance.
+ */
+function mapCacheSet(key, value) {
+  const data = getMapData(this, key);
+  const size = data.size;
+
+  data.set(key, value);
+  this.size += data.size == size ? 0 : 1;
+  return this;
+}
+
+export default mapCacheSet;