add support for deb822 sources

In Ubuntu 24.04, we are moving to deb822 sources by default, which is my main motivation for making these changes. These patches address setup-commands/setup-testbed, usage of $AUTOPKGTEST_APT_SOURCE_* in tools/, and add general support for deb822 to the testbed itself (e.g. --apt-pocket, --add-apt-source, etc.).

For the changes in setup-commands/setup-testbed, the idea is that your testbed uses either /etc/apt/sources.list or /etc/apt/sources.list.d/$DISTRO_ID.sources for default sources. For the changes in lib/, the goal is to support mixing and matching .list and .sources within the testbed. This means making it tolerable to have a missing /etc/apt/sources.list, and adding logic to handle /etc/apt/sources.list.d/*.sources.