Move internal modules to “internal” folder.

.internal/mapCacheDelete.js

@@ -0,0 +1,18 @@
+import getMapData from './.internal/getMapData.js';
+
+/**
+ * Removes `key` and its value from the map.
+ *
+ * @private
+ * @name delete
+ * @memberOf MapCache
+ * @param {string} key The key of the value to remove.
+ * @returns {boolean} Returns `true` if the entry was removed, else `false`.
+ */
+function mapCacheDelete(key) {
+  const result = getMapData(this, key)['delete'](key);
+  this.size -= result ? 1 : 0;
+  return result;
+}
+
+export default mapCacheDelete;