Fix span in proc macro

[Sindbad-Walter]

Follow up of rust-lang/rust#130277 that was initially handled in #4259. The rust team pointed out that the macro was not correctly setting the span and proposed a fix, being the use of Span::mixed_site instead of coping directly the span of the field.

The #[allow(unused_qualifications)] is still here in the wrap_in_dummy_mod as it is hard to make sure the lint is correctly done in every proc macro.

[weiznich]

Thanks for opening this PR 🙏

The changes itself look fine to me, although I personally would prefer to address all potential other locations that somewhat use a span at once.

(Also as minor note: Would it be possible to squash all those commits?)

[weiznich]

I would rather prefer auditing the derive crate to ensure all usages of spans are encoded correctly. Yes that's a bit of work but it should fix all these issues at once.
The alternative is to remove this allow and start searching again if something pops up in some code.

[Ten0]

That seems like a lot of work for something that's seldom going to be noticed considering the warning is not enabled by default. It looks like it's going to be hard to maintain if we don't have it in our CI.

To make that practical to fix and maintain, it looks like syn's Spanned trait should have a span_mixed method that constructs this.