Allow non-toplevel modules in python environment (#1042)

test/tests/test_environments/python_src/foo.py

@@ -0,0 +1,2 @@
+def bar():
+    return 'THIS_IS_FOO_BAR'

test/tests/test_environments/python_src/main.py

@@ -0,0 +1,5 @@
+def main():
+    return 'THIS_IS_MAIN_MAIN'
+
+def func():
+    return 'THIS_IS_MAIN_FUNC'

test/tests/test_environments/python_src/sub_mod/altmain.py

@@ -0,0 +1,2 @@
+def entrypoint():
+    return 'THIS_IS_ALTMAIN_ENTRYPOINT'