Add and enable pyflakes3 on console_conf, subiquity, subiquitycore and tests.
Replace NOQA markers with __all__ declarations for module imports.
Replace some obvious deadcode that voilates flake8, separate pass for removing
dead code rather than commenting will be handled later.
the only really visible effect of all this is to make --dry-run
--machine-config foo show the network config from foo, not the machine
running subiquity. (The existing configs won't work, though)