fix(lint): std_instead_of_core false positives for core::io/MSRV

[bushrat011899]

Objective

• Fix false positive identified by @rktaxe in this comment on MSRV-aware std_instead_of_core #13158.
• Fix MSRV-aware std_instead_of_core #13158

Solution

Previously the lint had an exception for all instances of a stable item in an unstable module, primarily to allow certain intrinsics such as copy to be accessible. Instead, I check for the presence of rustc_allowed_through_unstable_modules to handle those exceptions, and allow the is_stable check within the lint to early out as soon as any part of its path is unstable.

I believe this was the last piece required to resolve #13158, and have added tests for all examples listed in the issue. If there are other examples of this lint failing, I'd greatly appreciate seeing them!

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Notes

• No AI tooling of any kind was used during the creation of this PR.
• Originally opened (in error) as fix(clippy): std_instead_of_core false positive for core::io rust#156164

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Please write a short comment explaining your change (or "none" for internal only changes)

changelog: fix certain false positives for [std_instead_of_core] for stable items in an unstable module (e.g., core::io::ErrorKind), and other MSRV-unaware issues.

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[github-actions]

Lintcheck changes for bc66086

Lint	Added	Removed	Changed
clippy::std_instead_of_alloc	1	0	1
clippy::std_instead_of_core	7	56	5

This comment will be updated if you push new changes

[rustbot]

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[bushrat011899]

It was a little more involved than I anticipated, but I believe I have resolved the false positive I was intending to, and also solved a couple of additional multi-import ones. I've added additional test cases for the lint which should catch any regressions. This also solved some MSRV issues too. For example, serde 1.0.204 had an MSRV of 1.31 and imports AtomicBool, AtomicI8, and AtomicU8 in a single line. Current clippy suggests importing all from core, even though AtomicI8 and AtomicU8 were only added to core in 1.34.0.

Because I was struggling to understand how the lint worked initially, I've refactored it probably more than it strictly had to be. However I believe it is now easier to understand (at least to my new-to-clippy eyes), with additional comments and internal documentation.

@rustbot ready