Port the python pipeline to the v2 engine

[stuhood]

This is an umbrella ticket to cover porting a version of the python backend ("pipeline") away from the v1 Task model and into v2 @rules: it is expected that it will have a lot of blockers, but also that this ticket itself should be broken up and accomplished in phases from the bottom up.

This document describes rational and milestones:
https://docs.google.com/document/d/1ZO9396dugdoJEsGW1U8hC_QOj5SbE4OmhTGSG5tJ04U/edit?usp=sharing

[stuhood]

Some feedback from @illicitonion which I'll incorporate into the above:

• Error reporting
• Finessing the rules UX (maybe a DSL, maybe a generator model, maybe something else)
• If rules get any more abstracted or complicated: A way of testing rules
• Debuggability of rules
• Fleshing out the existing node visualisation stuff to make wiring together rules easier when there are more of them (may end up being the same as debuggability)
• (Probably: Optimising node and graph size)
• Snapshot manipulation ?intrinsics

[stuhood]

I've renamed the existing Python v2 Task Pipeline project to cover only Milestone 1 from the linked document so that we can focus on completing the milestones in order.

[stuhood]

I believe that the https://github.com/pantsbuild/pants/projects/6 milestone will act as a much better umbrella for this effort, so I'm going to close this one in favor of just tracking the milestone.