As a temporary measure until OEM is refactored in the 25.10 cycle, we need to
ensure that Calamares and its necessary dependencies are installed on the
target, iff this is a stacked squashfs *and* it is an OEM install. We're doing
this in pkgselect instead of pkgselectprocess because pkgselect already knows
about stacked squashfses, it's what provides the final "additional install
list" to be considered by checkpackage-backend, and this is a temporary fix
anyway (LP: #2104343).
