wip: build xwayland as patched Debian package

This adds a new script mkrepo.sh which:

• creates a Debian repository with patched packages
• creates a Debian chroot tarball if necessary for building
• rebuilds all source packages that either are not built yet or have a newer version in unstable
• all source packages that have a file named after the source package in patches are built -- I put xwayland in there as an example

This approach:

• is faster than building using qemu emulation (because we cross-compile instead)
• we don't need to install build dependencies onto the reform system image
• we don't install anything into /usr/local
• we patch the official Debian package and thus keep the delta low and benefit from packaging improvements
• moving this solution to publishing the packages on https://mntre.com/reform-debian becomes easy

TODO:

• install an empty Protected:yes package onto the reform system image that prevents apt upgrade from upgrading the patched packages