pkgset: Ignore duplicated module builds
    
    If the module tag contains the same module build multiple times (because
    it's in multiple tags in the inheritance), Pungi will not process that
    correctly and try to include the same NSVC in the compose multiple
    times. That leads to a crash.
    
    This patch adds another step to the inheritance filter to ensure the
    result contains each module only once.
    
    JIRA: RHELCMP-12768
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>