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_explicit/bar/a.py

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

python/ql/test/experimental/library-tests/CallGraph-implicit-init/foo_explicit/foo_explicit.py

@@ -0,0 +1,2 @@
+def foo_explicit_func():
+    print("foo_explicit_func called")