To complement the last fix, ensure that Calamares ends up on the removal list
for non-stacked squashfses if and only if OEM is disabled, and Calamares shows
up on the install list only for stacked squashfses which have OEM enabled.
This nuanced approach ensures all potential combinations have the intended
functionality.
If checkpackage-backend does not exist in the target system, likely due to the
use of stacked squashfses, ensure it temporarily exists so it can be used in
the install process. This fixes installation of the Virtual Machine Manager
when selected on the Customize menu (LP: #2104243).
