python: add tests for module import

- `--max-import-depth=3` to give points-to a chance
- `not_root` dir to force namespace package logic
- add usage in `example.py` to get files extracted

python/ql/test/experimental/library-tests/CallGraph-implicit-init/foo/bar/a.py

@@ -1,3 +1,6 @@
 def afunc():
-  print("afunc called")
-  return 1
+    print("afunc called")
+    return 1
+
+from foo.foo import foo_func
+foo_func() # $ pt,tt="foo/foo.py:foo_func"