Use C2SP/wycheproof by import not 'go mod download'

[jas4711]

Hi. I'm considering packaging this module for Debian. We build everything offline, and this piece of code:

// downloadWycheproofTestVectors downloads the JSON test files from
// the Wycheproof repository with `go mod download -json` and returns the
// absolute path to the root of the downloaded source tree.
func downloadWycheproofTestVectors() (string, error) {
        path := "github.com/C2SP/wycheproof@" + wycheproofModVer
        cmd := exec.Command("go", "mod", "download", "-json", path)

This breaks in Debian. Couldn't you import the C2SP module just like some other go module?

Thanks,
Simon