Disable multiversion installation in package transactions
Normally, package management transactions support so-called "multiversion"
or "install-only" packages, where these packages are designed to be parallel
installable while still using the same name. This is mainly done with kernel
packages, as people usually want fallback kernels.
However, the design of transactional updates makes this feature irrelevant, since
we take full system snapshots and have the ability to boot into any of them at
any given time. Thus, we can tolerate having only one kernel package installed
in any given snapshot.