generate_packets.py: move formatting in Variant.get_send() to each individual part
Part of #44577. get_send() equivalent of #44598. Rather than assembling a format string and then filling in the data, format each individual part. Where not needed, drop the formatting step entirely.
Rebased patch on new version of #44563
Part of #44577. get_send() equivalent of #44598. Rather than assembling a format string and then filling in the data, format each individual part. Where not needed, drop the formatting step entirely.