Move internal modules to “internal” folder.

.internal/mapCacheHas.js

@@ -0,0 +1,16 @@
+import getMapData from './.internal/getMapData.js';
+
+/**
+ * Checks if a map value for `key` exists.
+ *
+ * @private
+ * @name has
+ * @memberOf MapCache
+ * @param {string} key The key of the entry to check.
+ * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+ */
+function mapCacheHas(key) {
+  return getMapData(this, key).has(key);
+}
+
+export default mapCacheHas;