Freeciv

Part of #43927. This is a meta-ticket tracking the task of replacing the list[Packet] passed around the bottom quarter of the script with a dedicated PacketsDefinition class, and moving most of the module-level functions involved in either creating it or producing code based on it into that new class as methods. For an idea of what the end result might look like, the prototype (or current state thereof) is on my github fork. (Unless you're reading this far enough in the future, when that branch will likely no longer exist.)

Issues:

• create / move parse code into class #45167
• factor out method to iterate by number #45168
• move all_caps_union() #45169
• move packet_functional_capability generation #45170
• move delta_stats_report() generation #45171
• move delta_stats_reset() generation #45172
• move packet_name() generation #45173
• move packet_has_game_info_flag() generation #45174
• move packet_handlers_fill_initial() generation #45175
• move packet_handlers_fill_capability() generation #45176
• move enum packet_type generation #45177