python.mk: use script to process RECORD for the first pass

RECORD is a CSV file [0] containing more data than our plist format
accepts. The original processing method used sed(1) exclusively,
but paths containing quotes, spaces and commas throw it off. Process
the CSV properly instead for the first pass.

References: https://packaging.python.org/en/latest/specifications/recording-installed-packages/#the-record-file [0]