fix: Do not stop visible_parent_map breadth-first search reaching children of #[doc(hidden)] modules

[zalanlevai]

The visible_parent_map query misses putting the child items of #[doc(hidden)] modules into the queue, resulting in the breadth-first search missing them entirely. This results in inconsistent path printing behavior affecting diagnostics: namely that re-exports nested deeply within #[doc(hidden)] modules are not considered as a possible fallback path. See the linked issue (#159880) for more details and an example of how this manifests in diagnostics.

This PR makes sure that #[doc(hidden)] parents in the fallback map are still searched as part of the breadth-first search.

Fixes #159880.

The two tests for this change are based on the inconsistent diagnostics shown in the issue:

• tests/ui/suggestions/suggest-path-through-direct-dep-crate/hidden-reexport-of-transitive-dep-item.rs captures the current, expected behavior in the already working base case where the hidden module is still recorded in the fallback map;
• tests/ui/suggestions/suggest-path-through-direct-dep-crate/hidden-reexport-of-nested-transitive-dep-item.rs is a regression test for the breadth-first search covering nested children of #[doc(hidden)] modules to populate the fallback map by testing whether the diagnostic prefers the hidden but accessible path through the direct dependency crate.

Related PRs/issues:

• Prefer suggestion paths which are not doc-hidden  #87349
• Name resolution is confusing when crates re-exports std items #153477

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[zalanlevai]

@rustbot label +A-diagnostics +A-visibility