rustdoc: Fix cross-crate links to reexported items

Cross crate links can target items which are not rendered in the documentation.
If the item is reexported at a higher level, the destination of the link (a
concatenation of the fully qualified name) may actually lead to nowhere. This
fixes this problem by altering rustdoc to emit pages which redirect to the local
copy of the reexported structure.

cc #14515
Closes #14137