fetchHex produces outputs that mix will accept

  };

  unpackCmd = ''

    tar -xf $curSrc contents.tar.gz

    tar -xf $curSrc contents.tar.gz CHECKSUM metadata.config

    mkdir contents

    tar -C contents -xzf contents.tar.gz

    mv metadata.config contents/hex_metadata.config

    # To make the extracted hex tarballs appear legitimate to mix, we need to

    # make sure they contain not just the contents of contents.tar.gz but also

    # a .hex file with some lock metadata.

    # We use an old version of .hex file per hex's mix_task_test.exs since it

    # is just plain-text instead of an encoded format.

    # See: https://github.com/hexpm/hex/blob/main/test/hex/mix_task_test.exs#L410

    echo -n "${pkg},${version},$(cat CHECKSUM | tr '[:upper:]' '[:lower:]'),hexpm" > contents/.hex

  '';

  installPhase = ''