checksum: Refactor creating checksum files
    
    Instead of iterating over the images metadata and appending the checksum
    to relevant files immediately, we should store them and write only once.
    
    This avoid an issue when the same image is mentioned in the metadata
    multiple times. This happens for source images that are listed under
    each binary arch.
    
    The unified isos script is updated to use the exact same logic and code.
    This also uncovered a problem with the metadata for debuginfo unified
    isos: their paths in metadata were incorrect, which lead to missing
    checksums.
    
    Fixes: https://pagure.io/pungi/issue/667
    Fixes: https://pagure.io/pungi/issue/668
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>