Rustdoc Json: Add tests for Reexports, and improve jsondocck
The two changes are orthognal, so you can land just one if you want, but the improved errors realy helped write the tests.
Notably does not have the case from #80664, but I want to have all the ajacent cases tested before starting work on that to ensure I dont break anything.
Improves #81359
cc ```@CraftSpider```
r? ```@jyn514```
```@rustbot``` modify labels: +A-testsuite +T-rustdoc +A-rustdoc-json
a98dc9b3e7