Determine whether to automatically aggregate and create a symbol publishing exclusion file

[mmitche]

The symbol publishing exclusion file in the VMR is currently the aggregate of arcade, roslyn, and runtime's. It's unlikely to change much. However, it may be better to automatically aggregate it at build time, keeping in mind that runtime will no longer have an official build in which to use its SPEF.

[mmitche]

Don't think this is worth it at this point.