- 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.

Python/exceptions.c

@@ -1647,6 +1647,8 @@ PyDoc_STRVAR(FutureWarning__doc__,
 "Base class for warnings about constructs that will change semantically "
 "in the future.");
 
+PyDoc_STRVAR(ImportWarning__doc__,
+"Base class for warnings about probable mistakes in module imports");
 
 
 /* module global functions */
@@ -1719,6 +1721,7 @@ PyObject *PyExc_SyntaxWarning;
 PyObject *PyExc_OverflowWarning;
 PyObject *PyExc_RuntimeWarning;
 PyObject *PyExc_FutureWarning;
+PyObject *PyExc_ImportWarning;
 
 
 
@@ -1818,6 +1821,8 @@ static struct {
   RuntimeWarning__doc__},
  {"FutureWarning", &PyExc_FutureWarning, &PyExc_Warning,
   FutureWarning__doc__},
+ {"ImportWarning", &PyExc_ImportWarning, &PyExc_Warning,
+  ImportWarning__doc__},
  /* Sentinel */
  {NULL}
 };