to do
/////

* Public API left to expose (w/ docs!)

    + abc.PyLoader.get_source
    + util.set_loader

* Implement InspectLoader for BuiltinImporter and FrozenImporter.

    + Expose function to see if a frozen module is a package.

* Remove ``import *`` from importlib.__init__.

* Remove __all__ from importlib._bootstrap.

* Add leading underscores to all objects in importlib._bootstrap that are not
  publicly exposed.

* Reorder importlib/_bootstrap.py so definitions are not in inverted order.

* Make sure that there is documentation *somewhere* fully explaining the
semantics of import that can be referenced from the package's documentation
(even if it is in the package documentation itself, although it might be best
in the language specification).
