dispatch: use versiontuple() We have a new generic function for this. Use it.

util: add versiontuple() for returning parsed version information We have similar code in dispatch.py. Another consumer is about to be created, so establish a generic function in an accessible location.

extensions: rename _ignore to _builtin, add descriptive comment It was previously not at all obvious what this was for. We also change it to a set to avoid iterating over an admittedly small list repeatedly at startup time.