gh-133956 fix bug where dataclass wouldn't detect ClassVar fields if ClassVar was re-exported from a module other than typing

[taminomara]

This PR continues work by @dzherb from #134073.

I've simplified the code and removed strict checks from _is_type. Supporting arbitrary levels of module nesting turned out to be beneficial for overall code complexity, so I've included it to this PR; I can send it as a separate PR with a separate issue, though.

Motivation for current implementation

I think we shouldn't assume anything about user-provided type annotations. All we know from annotation ty.ClassVar is that there is an object ty, and we're getting its attribute ClassVar. We shouldn't assume that ty is a module, nor that it doesn't implement some custom __getattr__ logic; instead, we should just do what the annotation tells us to do.

• Issue: dataclasses: Synthetic __init__ method on dataclass has broken ClassVar annotation under some conditions. #133956

[python-cla-bot]

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