python: change pytest pkg/__init__.py to only collect the __init__.py Module

Previously it would collect the entire package, but this is not what
users expect.

Refs #3749
Fixes #8976
Fixes #9263
Fixes #9313

testing/example_scripts/collect/package_init_given_as_arg/pkg/__init__.py

@@ -0,0 +1,2 @@
+def test_init():
+    pass

testing/example_scripts/collect/package_init_given_as_arg/pkg/test_foo.py

@@ -1,2 +1,2 @@
-def test():
+def test_foo():
     pass