Previously, rustdoc's LibEmbargoVisitor unconditionally visited the
child modules of an external crate. If a module re-exported its parent
via 'pub use super::*', rustdoc would re-walk the parent, leading to
infinite recursion.
This commit makes LibEmbargoVisitor store already visited modules in an
FxHashSet, ensuring that each module is only walked once.
Fixes#40936
