Add base-files (>= 13.4~) as alternative dependency for usr-is-merged

The dependency on usr-is-merged (>= 38~) ensures that we have a system that is usrmerged and no longer allows an opt-out mechanism. The base-files package provides the same guarantees since version 13.4, so add that as an alternative dependency. This allows the usr-is-merged package to be uninstalled in trixie.