get 'goal builddict' and 'tests/python/pants_test/tasks:builddict' working again

un-break/re-enable 'builddict' goal and its test

As part of this added a class function to BuildFileParser: report_registered_context()
Returns a dict of symbols defined in BUILD files. This dict is quite useful for code that spews docs about symbols defined in BUILD files.

somewhat sad, but tolerable: many target constructor sigs now look roughly like __init__(arg, kwargs):Super(...).__init__(args,kwargs) . Thus, several of our formerly-detailed function sig docs are now less informative "args, kwargs". Maybe if I dig around in pydoc code and/or other reflection-y stuff, there's some straightforward way to do something lovely that automagically accumulates a method signature up the inheritance tree. I won't try to figure that out today, though.

Testing Done:
a lot of looking at old-Pants output vs new-Pants output, reconciling differences

re-enabled a test. and got it working again.

./build-support/bin/ci.sh

Reviewed at https://rbcommons.com/s/twitter/r/362/