feat: add method to delete mod metadata

Also moves indexDir setting from LocalModUpdateTask -> ModFolderModel

launcher/minecraft/mod/ModFolderModel.h

@@ -108,11 +108,16 @@ public:
 
     bool isValid();
 
-    QDir dir()
+    QDir& dir()
     {
         return m_dir;
     }
 
+    QDir indexDir()
+    {
+        return { QString("%1/.index").arg(dir().absolutePath()) };
+    }
+
     const QList<Mod> & allMods()
     {
         return mods;