manifest claims two packages are both installed that conflict

[Steve Langasek]

Investigating LP: #2036199, found that https://cdimage.ubuntu.com/ubuntu-unity/daily-live/20230914/mantic-desktop-amd64.manifest lists grub-pc + grub-efi-amd64. This wasn't surprising as Julian has mentioned work happening in Debian to make these packages co-installable. But that work hasn't landed yet, the packages still conflict.

Yet both are listed in the manifest for the squashfs, which shouldn't be possible.

$ sudo mount /tmp/ubuntu-unity-mantic-desktop-amd64.iso /mnt/
$ sudo mount /mnt/casper/filesystem.squashfs /mnt/
$ sudo chroot /mnt dpkg -l | grep grub-pc
rc grub-pc 2.12~rc1-4ubuntu3 amd64 GRand Unified Bootloader, version 2 (PC/BIOS version)
ii grub-pc-bin 2.12~rc1-4ubuntu3 amd64 GRand Unified Bootloader, version 2 (PC/BIOS modules)
$

The fact that grub-pc was removed is a bug. The fact that a package is in removed but not purged state in the image is ALSO a bug. We should probably just assert whenever this is the case and fail the build.

[Michael Hudson-Doyle (mwhudson)]

The bit that was confusing to me about this is that dpkg-query --show includes removed but not purged files.

[Philip Roche (philroche)]

The fact grub-pc exists in the manifest is related to https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/2036195 which is being verified now.

There is also a case which we have encountered in the public cloud team of `rc` packages still being present in the manifest. We resolved this by calling the following at the end of every binary hook

```
purge_removed_packages(){
  # Ensure that any packages that were removed during the build are
  # purged from the image. For example if chrony is installed it will
  # remove systemd-timesyncd, but it will not purge it leaving config-files
  # on disk and also resulting in `dpkg-query --show` listing systemd-timesyncd
  # and thus present in the package manifest despite not actually being installed.
  # purge_removed_packages will ensure that it is purged.
  MOUNTPOINT="$1"
  env DEBIAN_FRONTEND=noninteractive chroot "${MOUNTPOINT}" apt-get purge --assume-yes "?config-files"
}
```

We likely need the same in live-build/livecd-rootfs