issue #121: fix breakage caused by a9c6c13

This actually addresses multiple problems:

* Single-file programs were broken, since the fix introduced in
  6931cc10c4 caused builtin_find_module()
  to start indicating __main__ can always be loaded locally. That's
  broken, and there might be more cases where the same problem will crop
  up.

  Since it was indicated __main__ could be loaded locally, the built-in
  import machinery was allowed to attempt that (since we remove __main__
  from sys.modules during bootstrap), which caused a safety check to
  fire in the bowels of Python:

      "Cannot re-init internal module %.200s"

* The check for presence of the whitelist was totally broken, since the
  whitelist is never an empty list. Therefore 'self' was being returned
  for every module, including extension modules like 'termios'.

I have hand-verified this does not break the fix for issue #113. I
looked at writing a test for that, but it requires a Docker container
(or similar) with an ancient version of Ansible installed. Will open a
separate ticket tracking this.

mitogen/core.py

@@ -453,11 +453,18 @@ class Importer(object):
         return 'Importer()'
 
     def builtin_find_module(self, fullname):
+        # imp.find_module() will always succeed for __main__, because it is a
+        # built-in module. That means it exists on a special linked list deep
+        # within the bowels of the interpreter. We must special case it.
+        if fullname == '__main__':
+            raise ImportError()
+
         parent, _, modname = fullname.rpartition('.')
         if parent:
             path = sys.modules[parent].__path__
         else:
             path = None
+
         fp, pathname, description = imp.find_module(modname, path)
         if fp:
             fp.close()
@@ -484,8 +491,9 @@ class Importer(object):
 
             # #114: explicitly whitelisted prefixes override any
             # system-installed package.
-            if self.whitelist and not is_blacklisted_import(self, fullname):
-                return self
+            if self.whitelist != ['']:
+                if any(fullname.startswith(s) for s in self.whitelist):
+                    return self
 
             try:
                 self.builtin_find_module(fullname)