Most _appliancedeps should be optdeps

Description:

(Following on from #6 (closed))

Hey, it turns out that most of the _appliancedeps can actually be optdeps/makedeps. This should solve quite a few gripes we've had over the years about the amount of deps. And seeing as libguestfs is now a dep of vmexec which is apparently aiming to be a key part of Arch infra, it would be good to get this right.

Key points:

• supermin is the component responsible for instantiating the VM appliance and it ignores non-existent packages
• the libguestfs API has the concept of Availability

In other words, the VM appliance can be missing bits at runtime and it doesn't really matter. As long as they are available at build time, they can be installed later as optdeps.

Note: tweaking the list of _applincedeps is a little fiddly, so I've hacked up a script to hopefully make it a bit easier. The script contains all the logic for tuning the list. Notably, there is a "not optional" section (currently btrfs-progs and dosfstools). I'll submit a draft MR which includes the script. PTAL and lemme know if it's a viable approach for Arch or not.

Additional Note: if we go down this path, some small changes to deps will also be required for guestfs-tools which I'll submit as a separate MR.

Additional info:

• package version(s): 1.56.1-1 + bb25dd3d