[RFC] Support repository sources

[woky]

This is probably out of scope as it has too many variables, but for me, pacapt could completely abstract distro-specific package managers if it would also support adding additional repositories.

For instance, sometimes I need to install Docker CE on Debian as described in https://docs.docker.com/install/linux/docker-ce/debian/. The steps are roughly equivalent to:

apt update
apt install apt-transport-https ca-certificates gnupg2 lsb-release

OS_ID=$(lsb_release -si | tr '[:upper:]' '[:lower:]')
OS_REL=$(lsb_release -sc)

repo_name=docker-ce
base_url=https://download.docker.com/linux

curl -sSL $base_url/$OS_ID/gpg | apt-key add -
echo "deb [arch=amd64] $base_url/$OS_ID $OS_REL stable" >/etc/apt/sources.list.d/$repo_name.list
apt update