Actually make sure build_packages (abuild, apk-tools, etc) are built first

Similar to !2362 (closed) this tries to do properly what !2356 (merged) failed at. It replaces the hack of inserting abuild as the first dependency of packages by actually inserting it into the build queue if it needs building (and the same for the other packages in pmb.config.build_packages).

Additionally, clean up a few other minor things in chroot. See commits for details.