doc: Python doc, don't use twitter/commons as example

Back when Pants' source code lived in twitter/commons, it made sense to use example code from there. Nowadays, not so much. Use Pants' own src/python/example instead.

Instead of having sample code in the doc, "include" from src/python/example; that's easier to keep correct.

Testing Done:
build-support/bin/publish_docs.sh

Reviewed at https://rbcommons.com/s/twitter/r/654/
Resurrected pants goal idea.

This mostly involved updating outdated references, and making utility functions to mimick functionality refactored long ago.

Source and Javadoc jars are not mapped in, and I'm not quite sure how to get that working again. I'd appreciate any input on that matter.

The code in idea_gen and ide_gen isn't very clean, because it wasn't very clean in the first place. It would be good to go back at some point and tidy it up.

Idea gen is also much more efficient than it used to be, thanks to the round manager: it now avoids compiling or jarring anything, which saves tons of time. It would still be nice to include a flag to optionally disable codegen, but I'm not sure how (un)trivial that is at this juncture.