fetchurl: add unpacked passthru

[pbsds]

some-package.src.unpacked is great convenience to have when you want to for example view the setup.py or pyproject.toml file in the src of a fetchPypi package.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[K900]

Isn't this just pkgs.srcOnly?

[pbsds]

Isn't this just pkgs.srcOnly?

Close, but the UX is way better:

$ nix-build . -A hello.src.unpacked
this derivation will be built:
  /nix/store/00b1nzgl2pdnxxardrbxv3349xvhcvz3-hello-2.12.1.tar.gz-unpacked.drv
building '/nix/store/00b1nzgl2pdnxxardrbxv3349xvhcvz3-hello-2.12.1.tar.gz-unpacked.drv'
Running phase: unpackPhase
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
source root is hello-2.12.1
$ head -n2 result/README
This is the README file for the GNU Hello distribution.
Hello prints a friendly greeting.  It also serves as a sample GNU

I've expanded the docs to make the difference more clear

[K900]

I feel like all the allowAliases stuff isn't necessary? Unless your concern is that it will get picked up by Hydra. Also, can we somehow use srcOnly to define this, just to not duplicate the logic?