lib.fetchers.normalizeHash: replace `""` with `lib.fake*`

      hNames = [ "hash" ] ++ hashTypes;

      hAttrs = lib.genAttrs hNames (lib.const {});

  };

  fakeH = {

    hash = lib.fakeHash;

    sha256 = lib.fakeSha256;

    sha512 = lib.fakeSha512;

  };

in rec {

  proxyImpureEnvVars = [

    Converts an attrset containing one of `hash`, `sha256` or `sha512`,

    into one containing `outputHash{,Algo}` as accepted by `mkDerivation`.

    An appropriate “fake hash” is substituted when the hash value is `""`,

    as is the [convention for fetchers](#sec-pkgs-fetchers-updating-source-hashes-fakehash-method).

    All other attributes in the set remain as-is.

    # Example

    ```nix

    normalizeHash { } { hash = lib.fakeHash; foo = "bar"; }

    normalizeHash { } { hash = ""; foo = "bar"; }

    =>

    {

      outputHash = lib.fakeHash;

            ;

          in

            removeAttrs args hNames // (optionalAttrs (h != null) {

              outputHash = h.value;

              outputHashAlgo = if h.name == "hash" then null else h.name;

              outputHash =

                if h.value == "" then

                  fakeH.${h.name} or (throw "no “fake hash” defined for ${h.name}")

                else

                  h.value;

            })

  ;