Eliminates _almost_ all of the bespoke type trackers found here. The
ones that remain do not fit easily inside the framework of API graphs
(at least, not yet), and I did not see any easy ways to clean them up.
They have, however, been rewritten to use `LocalSourceNode` internally,
which was the primary goal of this exercise.
I'm sure we could also clean up many of the inner modules given the more
lean presentation we have now, but this can wait for a different PR.
Because the replacement extension point now extends `API::Node`, I
modified the `toString` method of the latter to have an empty body.
The alternative would be to require everyone to provide a `toString`
predicate for their extensions, but seeing as these will usually be
pointing to already existing API graph nodes, this seems silly.
(This may be the reason why the equivalent method in the JS libs has
such an implementation.)
We were missing out on `None`, `True`, and `False` as these do not
appear as actual attributes of the `builtins` module in Python 3
(because they are elevated to the status of keywords there)
The simple solution, then, is to just always include them directly.
This also requires that we make this part of the modeling public, which I guess
is step we want to take eventually anyway!
I'm not quite sure whether the modules `Django::Views` and `Django::Forms` are
actually helpful, or whether we should just have their modules available as
`Django::View`, `Django::Form`, and `Django::Field`...
Call nodes are always local sources (specifically sources of the return
value of the call), and so inheriting from `LocalSourceNode` will have
no effect on results, but _should_ make it a bit more smooth to use the
API.
