__package__, it was used. This was incorrect since it could be set to None to
represent the fact that a proper value was unknown. Now None will trigger the
calculation for __package__.
Discovered when running importlib against test_importhooks.
attribute. Was throwing AttributeError before. Discovered when running
test_builtin against importlib.
This exception change is specific to importlib.__import__() and does not apply to
import_module() as it is being done for compatibility reasons only.
entries in sys.path_importer_cache. While this differs from semantics in how
__import__ works, it prevents any implicit semantics from taking hold with
users.
+ Ditch using arguments to super().
+ Ditch subclassing from object directly.
+ Move directory check out of chaining path hook to file path hook/finder.
+ Rename some classes to better reflect they are finders, not importers.
and relies much more on meta path finders to abstract out various parts of
import.
As part of this the semantics for import_module tightened up and now follow
__import__ much more closely (biggest thing is that the 'package' argument must
now already be imported, else a SystemError is raised).
