package snapd 2.58+22.04.1 failed to install/upgrade: installed snapd package post-removal script subprocess returned error exit status 1

Dear reporter. Do you know if the snapd package on your system was customized in any way? Was "snap set ..." used to, for example, allow non-/home home directories? Was snapd and snap packages used for a while or was this a relatively fresh install that was just purged?

We're asking because the error that is reported seems to suggest a more serious problem that we would like to understand and fix.

I was able to reproduce this with 22.04, snapd 2.61.1. This is what is reported by findmnt:

```
maciek@vm:~$ findmnt
TARGET SOURCE FSTYPE OPTIONS
/ /dev/vda2 ext4 rw,relatime,errors=remount-ro
├─/sys sysfs sysfs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/kernel/security securityfs securityfs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/fs/cgroup cgroup2 cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate,memory_recursiveprot
│ ├─/sys/fs/pstore pstore pstore rw,nosuid,nodev,noexec,relatime
│ ├─/sys/firmware/efi/efivars efivarfs efivarfs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/fs/bpf bpf bpf rw,nosuid,nodev,noexec,relatime,mode=700
│ ├─/sys/kernel/debug debugfs debugfs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/kernel/tracing tracefs tracefs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/fs/fuse/connections fusectl fusectl rw,nosuid,nodev,noexec,relatime
│ └─/sys/kernel/config configfs configfs rw,nosuid,nodev,noexec,relatime
├─/proc proc proc rw,nosuid,nodev,noexec,relatime
│ └─/proc/sys/fs/binfmt_misc systemd-1 autofs rw,relatime,fd=29,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=41325
│ └─/proc/sys/fs/binfmt_misc binfmt_misc binfmt_misc rw,nosuid,nodev,noexec,relatime
├─/dev udev devtmpfs rw,nosuid,relatime,size=1949512k,nr_inodes=487378,mode=755,inode64
│ ├─/dev/pts devpts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000
│ ├─/dev/shm tmpfs tmpfs rw,nosuid,nodev,inode64
│ ├─/dev/hugepages hugetlbfs hugetlbfs rw,relatime,pagesize=2M
│ └─/dev/mqueue mqueue mqueue rw,nosuid,nodev,noexec,relatime
├─/run tmpfs tmpfs rw,nosuid,nodev,noexec,relatime,size=397560k,mode=755,inode64
│ ├─/run/lock tmpfs tmpfs rw,nosuid,nodev,noexec,relatime,size=5120k,inode64
│ ├─/run/credentials/systemd-sysusers.service ramfs ramfs ...

We now believe this bug is understood and we are thinking about a way to address it. The root cause is that the per-distro purge script is unaware of the mounts that need to be stopped and has no way to enumerate them.

I've opened a related PR to the firefox snap which replaces the problematic interface with system-files https://github.com/canonical/firefox-snap/pull/53