Add `bcvk deps`

[cgwalters]

Right now we distribute a binary, but we have a bunch of external dependencies (podman, libvirt). The secure boot stuff depends on python3-virt-firmware etc as of recently.

One idea I had is at build time we capture our runtime dependencies (depending on the target OS/distro) and thus make it easy to do something like bcvk deps | xargs apt -y install.

Of course it would also make sense to have per-distro packages.