general: add an adhoc test for checking mixin behaviour with namespace packages and __init__.py hack

also use that hack in my.fbmessenger

misc/overlay_for_init_py_test/my/fbmessenger/all.py

@@ -0,0 +1,7 @@
+from my.fbmessenger import export
+from . import mixin
+
+
+def messages():
+    yield from mixin.messages()
+    yield from export.messages()

misc/overlay_for_init_py_test/my/fbmessenger/mixin.py

@@ -0,0 +1,2 @@
+def messages():
+    yield from ['1', '2', '3']