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
Previously, `JsonRenderer::after_krate` called `krate.version.clone()`.
The problem was it did that after the version was already moved into the
cache, so it would always be None. The fix was to get the version from
the cache instead.
Remove struct_type from union output
Also bumps the format number and adds a test
Rationale: It's illegal to have unions of the form `union Union(i32, f32);`, or `union Union;`. The struct_type field was recently removed from the rustdoc Union AST, at which time this field was changed to always just read "union". It makes sense to completely remove it, as it provides no information.
