code: func packagesListing(pfx string, pkgs []string, active string) []listingItem {
  	out := make([]listingItem, 0, len(pkgs))
  	return pathListing(pfx, pkgs, out, func(p string) listingItem {
  		return listingItem{
  			Href:   packagePageURL(p, ""),
  			Active: p == active,
  		}
  	})
  }
docs: // packagesListing formats a list of packages under 'pfx'.
  //
  // One of them can be hilighted as Active.