load failure in relation to reloads. Also expose
importlib.util.module_for_loader to handle all of the details of this along
with making sure all current loaders behave nicely.
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).
Brett Cannon
Benjamin Peterson