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

[hunshcn]

Before gazelle would leave generated py_library targets aroundeven when no files in a directory exist because X. With this change gazelle correctly handles this case.

[aignas]

Thanks for the PR! Could you add a test that ensures that we don't regress in the future and adjust the changelog notes please?

[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?

[aignas]

In general LGTM, thank you very much for the PR.

Could you please modify the PR description so that is more useful as a git commit message when people look at the history later?

Something like:
Before gazelle would leave generated py_library targets aroundeven when no files in a directory exist because X. With this change gazelle correctly handles this case.

Summary:

1. Change private attrs
2. Something else.