init: Filter comps for modular variants with tags
    
    Modular variants can either be specified by a list of modules, or by a
    list of Koji tags. In terms of comps preprocessing there should not be
    any difference between the two.
    
    Resolves: https://pagure.io/pungi/issue/1640
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>