generate_packets.py: Turn sequence-building loops into expressions
Part of #43927. There are a number of places where a loop with a relatively small body is used to build up a collection or string element by element. Many of these could be better served with a list/set/dict comprehension or a str.join() call.
alienvalkyrie 
Part of #43927. There are a number of places where a loop with a relatively small body is used to build up a collection or string element by element. Many of these could be better served with a list/set/dict comprehension or a str.join() call.