Convert to packer-plugin-sdk

All dependencies updated. Go version bumped to 1.18. HCLv2 fixed to
v2.14.1 due to hashicorp/packer-plugin-sdk#131.

Wait until target has domain name resolution

If apt-get -y runs before the target has finished initializing its
network it's going to fail to fetch some packages, but will report
success anyway. Run resolvectl query in a loop until it can resolve
deb.debian.org to reduce the likelihood of generating an image with
missing packages.

Builder name is now packer-builder-nspawn

Update APT cache with archives from target

You no longer have to pre-populate the APT cache to reuse downloaded
packages between repeated builds of the same image. After running
`apt-get install`, the plugin will copy new package archives from target
back into `cache_dir`.

Provision packages with non-interactive apt-get

Initial features:
- DEBIAN_FRONTEND=noninteractive to make debconf happy
- --no-install-recommends for smaller and more reproducible images
- add APT sources and upload GPG keys for them
- pre-populate APT cache
- apt-get clean on cleanup