title: CloudDataflow can not use "google.cloud.datastore" package?
body: <p>I want to put datastore with transaction on CloudDataflow.
  So, I wrote below.</p>
  
  <pre><code>def exe_dataflow():
  ....
    from google.cloud import datastore
    # call from pipeline
    def ds_test(content):
      datastore_client = datastore.Client()
  
      kind = 'test_out'
      name = 'change'
      task_key = datastore_client.key(kind, name)
  
      for _ in range(3):
          with datastore_client.transaction():
              current_value = client.get(task_key)
              current_value['v'] += content['v']
              datastore_client.put(task)
  
      # pipeline
  ....
        | 'datastore test' &gt;&gt; beam.Map(ds_test)
  </code></pre>
  
  <p>But, Error occured and log message was displayed as below.</p>
  
  <pre><code>(7b75e0ef2db229da): Traceback (most recent call last):
    File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/batchworker.py", line 582, in do_work
      work_executor.execute()
    File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/executor.py", line 167, in execute
      op.start()
    ...(SNIP)...
    File "/usr/local/lib/python2.7/dist-packages/dill/dill.py", line 767, in _import_module
      return getattr(__import__(module, None, None, [obj]), obj)
  AttributeError: 'module' object has no attribute 'datastore'
  </code></pre>
  
  <p>CloudDataflow can not use "google.cloud.datastore" package?</p>
  
  <p><strong>add 2018/2/28.</strong></p>
  
  <p>I add --requirements_file to MyOption</p>
  
  <pre><code>  options = MyOptions(flags = ["--requirements_file", "./requirements.txt"])
  </code></pre>
  
  <p>and I make requirements.txt</p>
  
  <pre><code>google-cloud-datastore==1.5.0
  </code></pre>
  
  <p>But, Another error occurred.</p>
  
  <pre><code>(366397598dcf7f02): Traceback (most recent call last):
    File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/batchworker.py", line 582, in do_work
      work_executor.execute()
    File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/executor.py", line 167, in execute
      op.start()
  ...(SNIP)...
    File "my_dataflow.py", line 66, in to_entity
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/__init__.py", line 60, in &lt;module&gt;
      from google.cloud.datastore.batch import Batch
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/batch.py", line 24, in &lt;module&gt;
      from google.cloud.datastore import helpers
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/helpers.py", line 29, in &lt;module&gt;
      from google.cloud.datastore_v1.proto import datastore_pb2
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore_v1/__init__.py", line 17, in &lt;module&gt;
      from google.cloud.datastore_v1 import types
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore_v1/types.py", line 21, in &lt;module&gt;
      from google.cloud.datastore_v1.proto import datastore_pb2
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore_v1/proto/datastore_pb2.py", line 17, in &lt;module&gt;
      from google.cloud.datastore_v1.proto import entity_pb2 as google_dot_cloud_dot_datastore__v1_dot_proto_dot_entity__pb2
    File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore_v1/proto/entity_pb2.py", line 28, in &lt;module&gt;
      dependencies=[google_dot_api_dot_annotations__pb2.DESCRIPTOR,google_dot_protobuf_dot_struct__pb2.DESCRIPTOR,google_dot_protobuf_dot_timestamp__pb2.DESCRIPTOR,google_dot_type_dot_latlng__pb2.DESCRIPTOR,])
    File "/usr/local/lib/python2.7/dist-packages/google/protobuf/descriptor.py", line 824, in __new__
      return _message.default_pool.AddSerializedFile(serialized_pb)
  TypeError: Couldn't build proto file into descriptor pool!
  Invalid proto descriptor for file "google/cloud/datastore_v1/proto/entity.proto":
    google.datastore.v1.PartitionId.project_id: "google.datastore.v1.PartitionId.project_id" is already defined in file "google/cloud/proto/datastore/v1/entity.proto".
  ...(SNIP)...
    google.datastore.v1.Entity.properties: "google.datastore.v1.Entity.PropertiesEntry" seems to be defined in "google/cloud/proto/datastore/v1/entity.proto", which is not imported by "google/cloud/datastore_v1/proto/entity.proto".  To use it here, please add the necessary import.
  </code></pre>