Dynamic modules don't necessarily have a __package__ attribute

When trying to serialize some of the dynamically generated modules that pytest generates, errors like these are encountered.

```
  File "/Users/mvanniekerk/miniconda3/envs/py36/lib/python3.6/pickle.py", line 476, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 366, in save_function
    self.save_function_tuple(obj)
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 494, in save_function_tuple
    itertools.chain(f_globals.values(), closure_values or ()),
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 388, in _save_subimports
    if isinstance(x, types.ModuleType) and x.__package__:
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/py/_apipkg.py", line 123, in __makeattr
    raise AttributeError(name)
AttributeError: __package__
```

After applying this change the resulting serialized objects work correctly

cloudpickle/cloudpickle.py

@@ -385,7 +385,7 @@ def _save_subimports(self, code, top_level_dependencies):
         """
         # check if any known dependency is an imported package
         for x in top_level_dependencies:
-            if isinstance(x, types.ModuleType) and x.__package__:
+            if isinstance(x, types.ModuleType) and hasattr(x, '__package__') and x.__package__:
                 # check if the package has any currently loaded sub-imports
                 prefix = x.__name__ + '.'
                 for name, module in sys.modules.items():