distro: add apkg support

Move packaging templates from distro/DISTRO to distro/pkg/DISTRO.

Change packaging templates distro/pkg/* to use {{ version }}.

Add scripts/make_dev_archive.sh to create dev archives.

Add apkg config: distro/config/apkg.toml

With these changes, apkg can be used to package Knot DNS.

howto build Knot DNS packages with apkg

Get apkg.

With apkg installed:

# once per system
apkg system-setup

git clone -b apkg https://gitlab.nic.cz/knot/knot-dns/
cd knot-dns
apkg build-dep
apkg build

Inspect results in pkg/.