extdom: plugin doesn't allow @ in group name
    
    Old implementation handles username and group names with
    one common call. Character @ is used in the call to detect UPN.
    
    Group name can legaly contain this character and therefore the
    common approach doesn't work in such case.
    
    Also the original call is less efficient because it tries to resolv
    username allways then it fallback to group resolution.
    
    Here we implement two new separate calls for resolving users and
    groups.
    
    Fixes: https://bugzilla.redhat.com/1746951
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>