Rewrite libpackagelistparser.

I was happy to ignore this as long as it wasn't being touched, but it's
been modified twice in the last year. Time to just throw it out and
rewrite it.

Also add some basic tests and put them in presubmit.

Bug: http://b/127686429
Test: new tests
Change-Id: Ie7e9406521291de0eab3138f55068cee6aaf365a