Make main package Conflicts+Obsoletes with -standalone- subpackages
    
    I'm not entirely sure if this is the right form...
    Is Conflicts? useful when we have Obsoletes?
    
    Seem to work OK. I tested:
    dnf --installroot=... install x86_64/systemd-standalone-sysusers-246.6-2.fc34.x86_64.rpm  x86_64/systemd-standalone-tmpfiles-246.6-2.fc34.x86_64.rpm
    → succeeds with a new installation
    → fails if the installroot already had systemd installed
    dnf --installroot=... install x86_64/systemd{,-libs,-pam}-246.6-2.fc34.x86_64.rpm noarch/systemd-noarch-246.6-2.fc34.noarch.rpm
    → uninstalls the two standalone packages