Add support for hashing go packages

[andrew]

https://github.com/foragepm/zipdigest

something like:

const {zipDigest} = require('zipdigest');
const tmp = require('tmp-promise');
const fs = require('fs-extra');

async function dirhash(url) {
  var dir = await tmp.dir()

  var zippath = dir.path + '/temp.zip'

  const res = await fetch(url);
  const fileStream = fs.createWriteStream(zippath);
  await new Promise((resolve, reject) => {
      res.body.pipe(fileStream);
      res.body.on("error", reject);
      fileStream.on("finish", resolve);
    });

  var digest = await zipDigest(zippath)

  await fs.remove(zippath)

  return digest
}

algo should be called dirhash