Move internal modules to “internal” folder.

.internal/MapCache.js

@@ -0,0 +1,34 @@
+import mapCacheClear from './.internal/mapCacheClear.js';
+import mapCacheDelete from './.internal/mapCacheDelete.js';
+import mapCacheGet from './.internal/mapCacheGet.js';
+import mapCacheHas from './.internal/mapCacheHas.js';
+import mapCacheSet from './.internal/mapCacheSet.js';
+
+/**
+ * Creates a map cache object to store key-value pairs.
+ *
+ * @private
+ * @constructor
+ * @param {Array} [entries] The key-value pairs to cache.
+ */
+class MapCache {
+  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 `MapCache`.
+MapCache.prototype.clear = mapCacheClear;
+MapCache.prototype['delete'] = mapCacheDelete;
+MapCache.prototype.get = mapCacheGet;
+MapCache.prototype.has = mapCacheHas;
+MapCache.prototype.set = mapCacheSet;
+
+export default MapCache;