Unity内部资源文件打包AssetBundle测试

Unity默认有2个内部资源文件分别是:
Library/unity default resources
Resources/unity_builtin_extra

在打包AssetBundle时,如果AB中有引用到这些的话,会直接把需要的内容打包到AB中,但是这样如果细分打包的话,每个小的包就会重复打包这些内部资源,最后整个包体就会变大.
今天想了想能不能直接将这2个资源直接添加到打包的AB中,从而能想其他外部资源一样让Unity自动依赖这些文件,减小包体呢?
于是做了如下测试.

private void BuildBultinRes()
    {
        string outputpath = "Assets/BuiltinRes/";
        if (Directory.Exists(outputpath))
        {
            Directory.Delete(outputpath, true);
        }
        Directory.CreateDirectory(outputpath);

        List buildlist = new List();

        //Library/unity default resources
        AssetBundleBuild lib = new AssetBundleBuild();
        lib.assetBundleName = "lib.unity3d";
        lib.assetNames = new string[] { "Library/unity default resources" };
        buildlist.Add(lib);

        //Resources/unity_builtin_extra
        AssetBundleBuild exres = new AssetBundleBuild();
        exres.assetBundleName = "exres.unity3d";
        exres.assetNames = new string[] { "Resources/unity_builtin_extra" };
        buildlist.Add(exres);

        //ui/launch.unity3d
        //Assets/GameData/UI/Prefab/Launch/Panel_launch.prefab
        AssetBundleBuild launch = new AssetBundleBuild();
        launch.assetBundleName = "launch.unity3d";
        launch.assetNames = new string[] { "Assets/GameData/UI/Prefab/Launch/Panel_launch.prefab" };
        buildlist.Add(launch);

        BuildAssetBundleOptions options = BuildAssetBundleOptions.DeterministicAssetBundle | BuildAssetBundleOptions.UncompressedAssetBundle | BuildAssetBundleOptions.AppendHashToAssetBundleName;
        AssetBundleManifest manifest = BuildPipeline.BuildAssetBundles(outputpath, buildlist.ToArray(), options, EditorUserBuildSettings.activeBuildTarget);
    }

结果打包可以正常打,但是并不会打出期望的lib.unity3d和exres.unity3d,而其他如launch.unity3d等正常外部文件可以打出,也就是说加入Library/unity default resources和Resources/unity_builtin_extra和正常不加结果是完全一样的.
所以不能通过这种方式减小包体积.
那么应该怎么去掉这些内部资源呢?不知道未来Unity官方会不会注意到这问题,提供一种官方的解决方式.