Move internal modules to “internal” folder.

.internal/ListCache.js

@@ -0,0 +1,34 @@
+import listCacheClear from './.internal/listCacheClear.js';
+import listCacheDelete from './.internal/listCacheDelete.js';
+import listCacheGet from './.internal/listCacheGet.js';
+import listCacheHas from './.internal/listCacheHas.js';
+import listCacheSet from './.internal/listCacheSet.js';
+
+/**
+ * Creates an list cache object.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [entries] The key-value pairs to cache.
+ */
+class ListCache {
+  constructor(entries) {
+    let index = -1;
+    const length = entries == null ? 0 : entries.length;
+
+    this.clear();
+    while (++index < length) {
+      const entry = entries[index];
+      this.set(entry[0], entry[1]);
+    }
+  }
+}
+
+// Add methods to `ListCache`.
+ListCache.prototype.clear = listCacheClear;
+ListCache.prototype['delete'] = listCacheDelete;
+ListCache.prototype.get = listCacheGet;
+ListCache.prototype.has = listCacheHas;
+ListCache.prototype.set = listCacheSet;
+
+export default ListCache;