If you create a project with gnome builder and port it blueprint and try to build it in flatpak you will get a compiler error that /org/gnome/Example/window.ui.
Setting the source_dir variable in meson fixes this.
`blueprint-compiler port` interactively ports a project to blueprint.
It will create the subproject wrap file, add it to .gitignore, decompile
your GtkBuilder XML files, emit code to copy and paste into your
meson.build file, and update POTFILES.in.
It can't quite handle all of the features the forward compiler can, so
it will skip those files.
