Rollup merge of #143590 - GuillaumeGomez:reexport-shadowing, r=lolbinary

Fix weird rustdoc output when single and glob reexport conflict on a name

Fixes rust-lang/rust#143107.

The problem was that the second reexport would overwrite the first, leading to having unexpected results. To fix it, I now group items by their original `DefId` and their name and keep tracks of all imports for this item (should very rarely be more than one though, and even less often more than 2).

cc `@lolbinarycat`

Author: matthiaskrgr