[#79259864] Convert VeeWee scripts to Packer

Convert our VeeWee scripts to Packer, which gives us more flexibility and allows us to create builds in parallel.

I have tested this for the dev VM and with `vagrant-govuk` for VirtualBox.

I don't have VMWare Fusion; can @bradleywright or @samjsharpe please test with that?
Fixed in d2367d8.