Add path flakeref variant

Unlike file://<path>, this allows the path to be a dirty Git tree, so

  nix build /path/to/flake:attr

is a convenient way to test building a local flake.

src/libexpr/primops/flake.hh

@@ -35,6 +35,7 @@ struct Flake
     FlakeRef ref;
     std::string description;
     Path path;
+    std::optional<uint64_t> revCount;
     std::vector<FlakeRef> requires;
     std::shared_ptr<FlakeRegistry> lockFile;
     Value * vProvides; // FIXME: gc