Allow expressions within -p

[tobiasBora]

With nix-shell, I can run:

$ nix-shell -p "(python312.withPackages (ps: [ps.picos]))" -p "(python313.withPackages (ps: [ps.picos]))"

but I cannot run:

$ nixpkgs-review pr 398351 -p "(python312.withPackages (ps: [ps.picos]))" -p "(python313.withPackages (ps: [ps.picos]))"                                                                                                   
  $ git -c fetch.prune=false fetch --no-tags --force https://github.com/NixOS/nixpkgs master:refs/nixpkgs-review/0 pull/398351/merge:refs/nixpkgs-review/1
remote: Enumerating objects: 7, done.
remote: Counting objects: 100% (4/4), done.
remote: Compressing objects: 100% (4/4), done.
remote: Total 7 (delta 0), reused 0 (delta 0), pack-reused 3 (from 1)
Dépaquetage des objets: 100% (7/7), 21.94 Kio | 2.19 Mio/s, fait.
Depuis https://github.com/NixOS/nixpkgs
   472b4108d146..4513971596cb  master     -> refs/nixpkgs-review/0
  $ git worktree add /home/leo/.cache/nixpkgs-review/pr-398351-17/nixpkgs e980bd7639e26f4d2362fd9f2df3ee6b8d9cff6f
Préparation de l'arbre de travail (HEAD détachée e980bd7639e2)
Mise à jour des fichiers: 100% (47312/47312), fait.
HEAD est maintenant à e980bd7639e2 Merge 894c9f8bd0ae7f3c661fcf91942eaf3023e22048 into 472b4108d146d56eafdedaa30bb9376c4d139f89
warning: ignoring the client-specified setting 'system', because it is a restricted setting and you are not a trusted user
warning: ignoring the client-specified setting 'system', because it is a restricted setting and you are not a trusted user
These packages do not exist:
(python312.withPackages (ps: [ps.picos])) (python313.withPackages (ps: [ps.picos]))
  $ git worktree remove -f /home/leo/.cache/nixpkgs-review/pr-398351-17/nixpkgs

As I get an error These packages do not exist. However, nixpkgs-review already allow stuff like:

$ nixpkgs-review pr 398351 -p python312Packages.picos

so it would be cool to allow more complex expressions here like in nix-shell, especially for languages/editors that need a wrapper.

See also motivations in #484