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/flakeref.hh

@@ -122,9 +122,14 @@ struct FlakeRef
         std::optional<Hash> rev;
     };
 
+    struct IsPath
+    {
+        Path path;
+    };
+
     // Git, Tarball
 
-    std::variant<IsFlakeId, IsGitHub, IsGit> data;
+    std::variant<IsFlakeId, IsGitHub, IsGit, IsPath> data;
 
     // Parse a flake URI.
     FlakeRef(const std::string & uri);