fix(gazelle): delete invalid py_library and use correct NonEmptyAttrs for py_*

[hunshcn]

1. visibility should not be a NonEmptyAttr
2. when python_generation_mode is set to package, if __init__.py is deleted, the py_library generated for this package before should be deleted automatically.

[hunshcn]

changelog has been added.

It requires two test part.

1. Existing tests to ensure that everything is normal.
2. Make a empty folder only has BUILD with a py_library rule generated by gazelle before. After gazelle, BUILD will be empty.

But I have no idea who to add test about 2.

[aignas]

For 2. you should be able to construct a test similar to how we have them in https://github.com/bazelbuild/rules_python/tree/main/gazelle/python/testdata

You can have the bazel file before running gazelle and after running gazelle, where the test framework will ensure that it works as you expect it to.

[hunshcn]

test added @aignas

[hunshcn]

ping @aignas

[hunshcn]

could we continue?