Building stand-alone flatpak manifests (i.e. from flathub) is not supported
Opening up a flathub manifest directory cloned from https://github.com/flathub/*, GB already detects the flatpak manifest (if it's in JSON format, see #520). But then it stops building at the "primary module", because it assumes that you want to build your current working copy of this module, instead of what’s defined in the manifest.
In short, currently, GB perfectly supports the "App developer" use case, but not the "Flathub packager" use case. The latter only has the flathub manifest which comes "stand-alone", i.e. without the app’s source tree.
!346 (closed) tried to support this "packager" use-case, but it was abandoned, because it didn't handle
git worktrees and a few other application developers’ needs correctly.
So, it would be nice if GB could support building stand-alone flatpak manifests. If auto-detecting this state isn’t so easy to implement, perhaps adding a switch to manually disable the stop-at-primary-module functionality would be acceptable?