code: function getSharedDepDirs(pkgs, hash) {
    return pkgs.map(({ name, dir }) => {
      if (hash[name]) {
        return dir;
      }
      return false;
    }).filter(Boolean);
  }
docs: getSharedDepDirs selects an array of shared and linked packages
  @param  {Array} pkgs   An array of dependencies
  @param  {Object} hash  A hash map of our deps
  @return {Array}        A filtered list of shared pkg dirs