Disallow aliases and bare throws in pkgs/top-level/all-packages.nix

[tomodachi94]

Disallow aliases and bare (unconditional) throws in pkgs/top-level/all-packages.nix, and perhaps other top-level attrsets that have a separate *-aliases file.

Historically, this has been manually enforced. A rule or two in nixpkgs-vet would catch this error early, preventing PRs such as these from being necessary:

• Move some removal notice to aliases nixpkgs#327929
• ao: move to aliases nixpkgs#347997
• hop-cli: move throw to top-level/aliases.nix nixpkgs#348033

This should likely be a ratchet check.