warnAlias -> lib.warnings.warnAlias

[PerchunPak]

warnAlias was added in #456527 exclusively to pkgs/top-level/aliases.nix but I think it will be useful in stdlib.

Also comes with tests!

#457349 is included in this PR, because tests fail otherwise.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

---

Add a 👍 reaction to pull requests you find important.

[emilazy]

I would rather not codify this hack as public API with stability promises; it’s a temporary convenience to work around awkward Nix behaviour and the interaction between the imperfect script’s simple logic and existing aliases affected by the lack of structure in our package set. I’d expect structured aliases to handle this differently.