Skip over modules that don't have a __file__ field.

Otherwise, we get this error in deferred modules:

Traceback (most recent call last):
File "devtools/cdbg/debuglets/python/imphook2.py", line 339, in _InvokeImportCallbackBySuffix
    mod_root = os.path.splitext(module.__file__)[0]
AttributeError: 'module' object has no attribute '__file__'

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185320164

Author: emrekultursay

src/googleclouddebugger/imphook2.py

@@ -320,7 +320,12 @@ def GetModuleFromName(name, path):
     nonempty_modules = (m for m in modules if m)
 
     for module in nonempty_modules:
-      mod_root = os.path.splitext(module.__file__)[0]
+      # TODO(emrekultursay): Write unit test to cover None case.
+      mod_file = getattr(module, '__file__', None)
+      if not mod_file:
+        continue
+
+      mod_root = os.path.splitext(mod_file)[0]
 
       # If the module is relative, add the curdir prefix to convert it to
       # absolute path. Note that we don't use os.path.abspath because it