InstallerDiskOps::generate_fstabs only supports encryption with LVM

[tobyc11]

Distribution (run cat /etc/os-release):
Pop!_OS

Related Application and/or Package Version (run apt policy $PACKAGE NAME):
distinst

Issue/Bug Description:
Installing the OS with an ext4 formatted and luks encrypted root filesystem will cause distinst to generate empty crypttab, making the newly installed system unbootable.

Steps to reproduce (if you know):

cryptsetup luksFormat /dev/sda3
cryptsetup luksOpen /dev/sda3 cryptroot
# Instead of using LVM, we format a plain ext4 inside the luks encrypted block device:
mkfs.ext4 /dev/mapper/cryptroot

Proceed with installation where the previously formatted volume is mounted at /.

Expected behavior:
When installation finishes, crypttab is empty and the system is unbootable.

Other Notes:
I'll probably take a look into this myself when I become freer.