Add support for local package repositories

live-build has been able to install packages from a local deb repo for a while now, but this functionality simply installs everything placed under the packages.chroot folder. This is unsuitable for usecases where one only wants to install specific local packages, allowing their dependencies to be marked as automatically installed. The workaround of hosting a deb repo is both difficult and unsuitable for usecases where an end-user is expected to be able to build all local packages from source and then use them to build the final ISO.

To support this edge case, allow the user to point live-build to one or more local apt repositories by passing their paths to lb_config with a --chroot-localrepos argument. live-build will bind-mount the specified directories into the chroot, install temporary sources.list files for them, and then proceed with package installation, allowing those packages to be used like any other apt repo. The sources.list files for these temporary files are deleted and the bind-mounts removed once they are no longer necessary.