It is currently not possible to build Squeekboard with only packages that are available in Debian. This is not likely to change until Squeekboard has been ported to GTK4 or gtk4-layer-shell. Until then, vendor the dependencies during the build, so that one can still use the ".deb"-packages for testing. Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/659>