builder: Don't download modules that won't be built

Currently flatpak-builder downloads the sources for all modules, even if
the --stop-at option prevents them from being built. This commit changes
the behavior so that we don't bother to download the ignored modules.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f