package-repositories repeatedly adds identical entries to sources.list between runs

[Colin Watson]

The implementation of `package-repositories` in `lpcraft.commands.run._install_apt_packages` appends entries to `/etc/apt/sources.list` without checking whether they're already present. As a result, if you repeatedly run `lpcraft` locally against the same tree without running `lpcraft clean` in between, the container's `/etc/apt/sources.list` keeps growing and you get warnings from apt.

I think it would make sense to write additional apt repositories to a separate `/etc/apt/sources.list.d/lpcraft.list` file. It would then be reasonably straightforward to ensure that that file only contains the additional apt repositories specified in the current run, or to remove that file if the current run doesn't specify any additional apt repositories. (Replacement apt repositories should continue to write to `/etc/apt/sources.list` as before.)