Add a 'zonMeta' attribute to output nix expression

[icetan]

Add a passthru attribute to the link farm derivation which contains metadata from the zon-file. Useful for setting package name and version in one place.

It doesn't break the current interface as the extra metadata is accessible on the derivation:

deps = callPackage ./deps.nix { };
pkg = mkDerivation {
  name = "${deps.zonMeta.name}-${deps.zonMeta.version}";
  postPatch = ''
    ln -s ${deps} $ZIG_GLOBAL_CACHE_DIR/p
  '';
  ...
};

Input build.zig.zon:

.{
    .name = "zink",
    .version = "0.0.2",
    .license = "MIT",
    .author = "icetan",
    .dependencies = .{
        .@"glob-zig" = .{
            .url = "https://github.com/icetan/glob-zig/archive/main.tar.gz",
            .hash = "122009122341200514961d7b8215b07ef5b9ff4db61eeb5cd2a4949716d14761373a",
        },
    },
    .paths = .{
        "build.zig",
        "build.zig.zon",
        "src",
    },
}

Results in Nix output:

{ linkFarm, fetchzip }:

(linkFarm "zig-packages" [
  {
    name = "122009122341200514961d7b8215b07ef5b9ff4db61eeb5cd2a4949716d14761373a";
    path = fetchzip {
      url = "https://github.com/icetan/glob-zig/archive/main.tar.gz";
      hash = "sha256-OVRHNvp5F/HRwAIFOah1r7aMnNNBctXpGsY7cOh3MsA=";
    };
  }
]).overrideAttrs {
  passthru.zonMeta = {
    "author" = "icetan";
    "license" = "MIT";
    "name" = "zink";
    "version" = "0.0.2";
  };
}

For now only metadata that is a string will be added. In the future .paths might be nice to convert to nix paths to use in source filters.

Hope this is useful :)