pkgset: Support extra builds with no tags
    
    This is a rather fringe use case. If the configuration contains
    pkgset_koji_builds or pkgset_koji_scratch_tasks but no pkgset_koji_tag,
    the compose will be empty.
    
    The expectation though is that the packages should be pulled.
    
    The extra RPMs are added to all non-modular tags because they are
    supposed to mask builds from the same packages (e.g. user may want to
    explicitly pull in older version than tagged).
    
    This patch adds support for composes containing only explicitly listed
    builds by creating a dummy package set that is not actually using any
    tag.
    
    JIRA: RHELCMP-11385
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>