extract from default.nix

default.nix

{
  # ...
  
  src =
    let
      filterFn = path: type: lib.strings.hasPrefix (builtins.toString ./Celeste.Mod.mm) path
                          || lib.strings.hasPrefix (builtins.toString ./MiniInstaller ) path
                          || lib.strings.hasPrefix (builtins.toString ./lib           ) path;
    in
      builtins.filterSource filterFn ./.;
  
  # ...
}