fix(clippy): std_instead_of_core false positive for core::io

[bushrat011899]

Objective

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

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Notes

• No AI tooling of any kind was used during the creation of this PR.

[rustbot]

The Clippy subtree was changed

cc @rust-lang/clippy

[rustbot]

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