php: Refactor to have the different versions as separate files

[etu]

Motivation for this change

This should enable easier auto updates of the packages.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[aanderse]

@etu this change looks good 👍 but I haven't had a chance to look in detail what the bot has caught.

[etu]

It seems like the overrides broke, because it's unit that complains because it attempts to override things...

[etu]

Some elipsises in the function declarations seems fix it, then I got the same output of unit as before these changes.

This wasn't needed before because we called the default.nix with a very generic set of arguments, then we got the attribute back and did the override on it. Now we call the attribute more directly instead of inheriting it from the output of another function.