generate_packets.py: Make output paths for each file optional
Doing this properly is nontrivial. Just making the positional arguments optional would mean it's only possible to omit the later arguments, not the earlier ones. We need to add --name options for each path.
There are two ways we could go about this:
(I assume one can tell from my choice of words which one I'd prefer, but to spell it out – unless we desperately need backwards compatibility here, I'd go with the second option.)
There's also the question of whether we'd still allow passing empty strings as an alternative to omitting arguments; IMO that decision should mirror the first one (i.e. keep allowing it only if we choose to maintain backwards compatibility).
Attached patch takes the second option, i.e. breaks backwards compatibility for potential custom builds. If there is any reason why we need to maintain backwards compatibility here, now would be a good time to mention it.
Part of #43927. Followup to #43971. Sometimes (like current CI meson build) it might be desirable to only write some, not all, of the output files. This should be officially supported by making the respective path arguments optional, rather than by passing empty paths.