Provide 'types' and 'enums' submodule view (#273)

This is a minor hack to preserve the module interface from the
monocode: types and enums for a service live in distinct submodules.

Includes tests for enums

gapic/templates/%namespace/%name_%version/%sub/enums.py.j2

@@ -0,0 +1,13 @@
+{% extends '_base.py.j2' %}
+
+{% block content %}
+{% for p in api.protos.values() if p.file_to_generate and p.enums -%}
+from .types.{{ p.module_name }} import ({% for e in p.enums.values() %}{{ e.name }}, {% endfor %})
+{% endfor %}
+
+__all__ = (
+{%- for p in api.protos.values() if p.file_to_generate %}{% for e in p.enums.values() %}
+    '{{ e.name }}',
+{%- endfor %}{% endfor %}
+)
+{% endblock %}

gapic/templates/%namespace/%name_%version/%sub/types/__init__.py.j2

@@ -0,0 +1,13 @@
+{% extends '_base.py.j2' %}
+
+{% block content %}
+{% for p in api.protos.values() if p.file_to_generate and p.messages -%}
+from .{{ p.module_name }} import ({% for m in p.messages.values() %}{{ m.name }}, {% endfor %})
+{% endfor %}
+
+__all__ = (
+{%- for p in api.protos.values() if p.file_to_generate %}{% for m in p.messages.values() %}
+    '{{ m.name }}',
+{%- endfor %}{% endfor %}
+)
+{% endblock %}

gapic/templates/tests/unit/%name_%version/%sub/test_%service.py.j2

@@ -20,6 +20,9 @@ from google.api_core import future
 from google.api_core import operations_v1
 from google.longrunning import operations_pb2
 {% endif -%}
+{% if api.protos.values()|selectattr('enums') -%}
+from {{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }} import enums
+{% endif -%}
 {% for method in service.methods.values() -%}
 {% for ref_type in method.ref_types
    if not ((ref_type.ident.python_import.package == ('google', 'api_core') and ref_type.ident.python_import.module == 'operation')
@@ -431,4 +434,13 @@ def test_{{ message.name|snake_case }}_path():
 
 {% endwith -%}
 {% endfor -%}
+
+{% with enums = [] -%}{% for p in api.protos.values() if p.file_to_generate -%}{% for e in p.enums.values() if e.ident.module == service.meta.address.module -%}{% do enums.append(e) %}
+{% if enums -%}
+def test_enum_path():
+{%- for e in enums %}
+    assert enums.{{ e.name }} == {{ e.ident.module }}.{{ e.name }}
+{%- endfor %}
+{% endif -%}
+{% endfor -%}{% endfor -%}{% endwith -%}
 {% endblock %}