Define __all__ (#625)

* Define __all__

* Use tuple instead of list

* Add test

tests/test_all_definition.py

@@ -0,0 +1,19 @@
+def test_all_definition():
+    """
+    Check that a compiled module defines __all__ with the right value.
+
+    These modules have been chosen since they contain messages, services and enums.
+    """
+    import tests.output_betterproto.enum as enum
+    import tests.output_betterproto.service as service
+
+    assert service.__all__ == (
+        "ThingType",
+        "DoThingRequest",
+        "DoThingResponse",
+        "GetThingRequest",
+        "GetThingResponse",
+        "TestStub",
+        "TestBase",
+    )
+    assert enum.__all__ == ("Choice", "ArithmeticOperator", "Test")