Require aliases for submodule imports

[DavidCEllis]

Submodule imports without aliasess were rather messy and the general logic is much improved by their removal.

They were always limited in that any use of the main module was forced to import all submodules that could be used as there is no way for the class to know what attributes are being accessed.

eg:

from ducktools.lazyimporter import ModuleImport, LazyImporter

laz = LazyImporter([ModuleImport("importlib.util"), ModuleImport("importlib.machinery")])

laz.importlib.machinery  # importlib, importlib.util and importlib.machinery are all imported here.

Now all of the import subclasses require 'asname' or 'asnames' to be defined.