expose lib as a functor

This allows calling the nix-filter input directly. Eg:

    outputs = { self, nix-filter }:
      ...
      src = nix-filter {
        root = self;

        includes = [
          ./dir
        ];
      };

flake.nix

@@ -2,6 +2,7 @@
   description = "nix-filter";
 
   outputs = { self }: {
+    __functor = self.lib.__functor;
     lib = import ./default.nix;
     overlays.default = _: _: { nix-filter = self.lib; };
   };