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This mostly involved updating outdated references, and making utility functions to mimick functionality refactored long ago.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.
This was the problem where we would specify a jvm_binary with a source='...AppMain' in addition to depending on a lib which included those sources, which would mess up the source root. I know we fixed that particular pattern but I'm not confident we've completely eliminated that class of problems. 

However, I suppose it is a kind of 'broken' thing to support, and the other pants-devs probably won't like that, so perhaps it'd be best just to remove this.