Add flatpak-build subcommand
    
    The flatpak-build subcommand starts a build of the current git
    repository as a Flatpak. It's pretty much identical to container-build,
    but the build target is determined by looking up the module that will
    be built into the flatpak in container.yaml and then determining what
    platform version that uses by traversing module dependencies.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>