"Pack all" removes unrelated attributes

```nix { a.x = 1; b.y = 2; a.z = 3; } ``` Running "Pack all 'a' bindings to nested set" yields: ```nix { a = { x = 1; z = 3; }; } ``` Notice that `b.y = 2` was removed. Version: /nix/store/31ns7kqnsw90p8knh44sjn9gqibjs4bi-nixd-2.9.1