This reverts commit cac0cb7f02.
This doesn't work since CI currently mixes flatpak-builder and the host git repo
and doesn't download the gtk sources, so all subprojects are missing.
Instead of building the projects from my forks, build them as gtk
subprojects.
To avoid meson hitting the network for those wraps, add the required
files as extra sources and put them into subprojects/packagecache,
so meson can find them at build time.
