fetchurl: add passthru resolvedUrl (#398843)

      }

    else

      throw "fetchurl requires a hash for fixed-output derivation: ${lib.generators.toPretty urls_}";

  resolvedUrl =

    let

      mirrorSplit = lib.match "mirror://([[:alpha:]]+)/(.+)" url;

      mirrorName = lib.head mirrorSplit;

      mirrorList =

        if lib.hasAttr mirrorName mirrors then

          mirrors."${mirrorName}"

        else

          throw "unknown mirror:// site ${mirrorName}";

    in

    if mirrorName == null then url else "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";

in

assert

    inherit meta;

    passthru = {

      inherit url;

      inherit url resolvedUrl;

    }

    // passthru;

  }