Use Python namespace packages

Before, namespace packages would break in auditwheel due to a
superfluous check (fixed in the commit referenced in the Dockerfiles).
Now, the auditwheel used in the manylinux1-support Docker images can
handle namespace packages, and we may thus use them. This should
alleviate future user pain w.r.t. installation of grpcio-tools causing
loss of grpcio files and vice versa (e.g. requiring a reinstall of
grpcio following installation of grpcio-tools).

setup.py

@@ -234,8 +234,7 @@ setuptools.setup(
   ext_modules=CYTHON_EXTENSION_MODULES,
   packages=list(PACKAGES),
   package_dir=PACKAGE_DIRECTORIES,
-  # TODO(atash): Figure out why auditwheel doesn't like namespace packages.
-  #namespace_packages=['grpc'],
+  namespace_packages=['grpc'],
   package_data=PACKAGE_DATA,
   install_requires=INSTALL_REQUIRES,
   setup_requires=SETUP_REQUIRES,

tools/distrib/python/grpcio_tools/setup.py

@@ -88,8 +88,7 @@ setuptools.setup(
       protoc_ext_module(),
   ]),
   packages=setuptools.find_packages('.'),
-  # TODO(atash): Figure out why auditwheel doesn't like namespace packages.
-  #namespace_packages=['grpc'],
+  namespace_packages=['grpc'],
   install_requires=[
     'protobuf>=3.0.0a3',
   ],

tools/dockerfile/grpc_artifact_python_manylinux_x64/Dockerfile

@@ -41,3 +41,10 @@ RUN /opt/python/cp27-cp27mu/bin/pip install cython
 RUN /opt/python/cp34-cp34m/bin/pip install cython
 RUN /opt/python/cp35-cp35m/bin/pip install cython
 
+####################################################
+# Install auditwheel with fix for namespace packages
+RUN git clone https://github.com/pypa/auditwheel /usr/local/src/auditwheel
+RUN cd /usr/local/src/auditwheel && git checkout bf071b38c9fe78b025ea05c78b1cb61d7cb09939
+RUN /opt/python/cp35-cp35m/bin/pip install /usr/local/src/auditwheel
+RUN rm /usr/local/bin/auditwheel
+RUN cd /usr/local/bin && ln -s /opt/python/cp35-cp35m/bin/auditwheel

tools/dockerfile/grpc_artifact_python_manylinux_x86/Dockerfile

@@ -41,3 +41,10 @@ RUN /opt/python/cp27-cp27mu/bin/pip install cython
 RUN /opt/python/cp34-cp34m/bin/pip install cython
 RUN /opt/python/cp35-cp35m/bin/pip install cython
 
+####################################################
+# Install auditwheel with fix for namespace packages
+RUN git clone https://github.com/pypa/auditwheel /usr/local/src/auditwheel
+RUN cd /usr/local/src/auditwheel && git checkout bf071b38c9fe78b025ea05c78b1cb61d7cb09939
+RUN /opt/python/cp35-cp35m/bin/pip install /usr/local/src/auditwheel
+RUN rm /usr/local/bin/auditwheel
+RUN cd /usr/local/bin && ln -s /opt/python/cp35-cp35m/bin/auditwheel