`launch.py` import times are slow

[samuelstevens]

@MisteFr reports slow launch times on the ETH cluster when running uv run scripts/launch.py shards .... I suspect this is a problem because it needs to import every subcommand's package.

There are some issues with directly running modules in src/saev/framework because of submitit + cloudpickle + jaxtyping (see this open issue) but perhaps there is a better way to do this.