[refactor] Use proto-plus 0.3.0.

This moves from `Meta` inner classes to a  property on the module,
which is an overall improvement (not repeating the same thing over and over,
no weird `__all__` magic).

Author: Luke Sneeringer

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/types/$proto.py.j2

@@ -14,20 +14,30 @@ import proto{% if p != 'proto' %} as {{ p }}{% endif -%}
 {{ import_ }}
 {% endfor %}
 
-__all__ = (
+__protobuf__ = {{ p }}.module(
+    package='{{ '.'.join(proto.meta.address.package) }}',
+    {% if api.naming.proto_package != '.'.join(proto.meta.address.package) -%}
+    marshal='{{ api.naming.proto_package }}',
+    {% endif -%}
+    manifest={
     {%- for enum in proto.top.enums.values() %}
-    '{{ enum.name }}',
-    {%- endfor -%}
+        '{{ enum.name }}',
+    {%- endfor %}
     {%- for message in proto.top.messages.values() %}
-    '{{ message.name }}',
+        '{{ message.name }}',
     {%- endfor %}
+    },
 )
 
+
 {% for enum in proto.top.enums.values() -%}
     {% include '$namespace/$name_$version/$sub/types/_enum.py.j2' with context %}
 {% endfor %}
 
 {% for message in proto.top.messages.values() -%}
     {% include "$namespace/$name_$version/$sub/types/_message.py.j2" with context %}
 {% endfor %}
-{% endwith %}{% endblock %}
+{% endwith %}
+
+__all__ = tuple(sorted(__protobuf__.manifest))
+{% endblock %}

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/types/_message.py.j2

@@ -20,8 +20,4 @@ class {{ message.name }}({{ p }}.Message):
     {% endif %})
     """{{ field.meta.doc|rst(indent=4) }}"""
     {% endfor %}
-
-    class Meta:
-        package = '{{ message.ident.proto_package }}'
-        marshal = '{{ api.naming.warehouse_package_name }}'
 {{ '\n\n' }}

packages/gapic-generator/gapic/templates/setup.py.j2

@@ -22,7 +22,7 @@ setuptools.setup(
         'google-api-core >= 1.3.0, < 2.0.0dev',
         'googleapis-common-protos >= 1.6.0b6',
         'grpcio >= 1.10.0',
-        'proto-plus >= 0.2.1',
+        'proto-plus >= 0.3.0',
     ),
     classifiers=[
         'Development Status :: 3 - Alpha',