lug: apt mirror script

[PhotonQuantum]

As per sjtug/mirror-requests#79 (comment), we need an apt-mirror script to sync apt repositories from HTTP upstream.

An example config:

source: https://apt.pop-os.org
name: popos
arch: all,arm64,amd64
repo: release:jammy:main,release:impish:main
path: /some/path/popos
apt_mirror_path: /some/path/apt_mirror_path

mirror, skel and var directories will be created under apt_mirror_path, and the mirror directory will be symlinked to path. apt_mirror_path should never be cleaned manually, which apt-mirror handles.

Potential problem: does Caddy follow root path symlink? Say caddy is serving /some/path while /some/path itself is a symlink to another directory.