Use single dead annotation if entire module is unused

[sim642]

Running reanalyze 389dd68 on Goblint goblint/analyzer@a544002 revealed the following.

If an entire module is unused, then every single definition in the module is marked with a dead annotation (using -write at least). Although true, I think it's a bit too verbose.
It would be neat if there could be just one annotation for the entire module.