Ignore #[doc(hidden)] items when computing trimmed paths for diagnostics

[Zalathar]

The trimmed_def_paths query examines all items in the current crate, and all pub items in immediate-dependency crates (including the standard library), to see which item names are unique and can therefore be printed unambiguously as a bare name without a module path.

Currently that query has no special handling for #[doc(hidden)] items, which has two consequences:

• Hidden names can be considered unique, and will therefore be printed without a path, making it hard to find where that name is defined (since it normally isn't listed in documentation).
• Hidden names can conflict with visible names that would otherwise be considered unique, causing diagnostics to mysteriously become more verbose based on internal details of other crates.

This PR therefore makes the trimmed_def_paths query ignore external-crate items that are #[doc(hidden)], along with their descendants.

As a result, hidden item names are never considered unique for trimming, and no longer interfere with visible item names being considered unique.

---

• Fixes Standard library internal modules are treated as potential name colliders #148387.

[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[rustbot]

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