Right now, the policy pb2.py protobuf compilation is implemented as dependency in client/deps/policy_protos. Because dependencies are installed at runtime using self.job.install_pkg, the protobufs cannot be imported in the header of the .py files that use them. Figure out a way to move protobuf installation to another stage so that the imports can be moved up.

Moreover, policy_testserver.py in the Chromium code base expects the proto path to be in the import paths, see
https://cs.chromium.org/chromium/src/chrome/browser/policy/test/policy_testserver.py?q=policy_testserver.py&sq=package:chromium&dr=C&l=87
Clean this up.