Making sha256 or sha1 sum for http_archive optional

I have been trying to create a tool like reindeer but for go modules. But I've hit a barrier where creating the http_archive rules for the required modules requires the sha sum of the file it is downloading. However there is no easy way to get the hash of the files without downloading them. go.sum files or go sum db has some hashes but they are not the hash of the zip file rather a directory hash function using the actual contents of the file. One way to do this would be to download everything with the tool calculate the hashes and then generate the rules for it. Is there a way to circumvent this ?