go-macos-pkg was built out of a desire to generate and sign macOS pkgs on non-macOS OSs.

• Right now most of the heavy lifting is done by calling the xar utility, as there aren't currently (2021) Go libraries that write xar archives. The good news is xar is available on other OSs, unlike productsign/productbuild
• Right now only payload-free (e.g. just runs postinstall) packages can be generated as that's all I currently need. It shouldn't be too hard for someone to add payload capabilities, though.

script := []byte("#!/bin/bash\necho 'hello, world!'\n")

pkg, err := macospkg.GeneratePkg("com.github.korylprince.go-macos-pkg", "1.0.0", script)
if err != nil {
    log.Fatalln("could not generate pkg:", err)
}

// use Apple Developer ID Installer cert and key to sign
signed, err := macospkg.SignPkg(pkg, cert, key)
if err != nil {
    log.Fatalln("could not sign pkg:", err)
}

if err = os.WriteFile("signed.pkg", signed, 0644); err != nil {
    log.Fatalln("could not write signed pkg:", err)
}