- Add new Warning class, ImportWarning

 - Add new Warning class, ImportWarning

 - Warn-raise ImportWarning when importing would have picked up a directory
   as package, if only it'd had an __init__.py. This swaps two tests (for
   case-ness and __init__-ness), but case-test is not really more expensive,
   and it's not in a speed-critical section.

 - Test for the new warning by importing a common non-package directory on
   sys.path: site-packages

 - In regrtest.py, silence warnings generated by the build-environment
   because Modules/ (which is added to sys.path for Setup-created modules)
   has 'zlib' and '_ctypes' directories without __init__.py's.