rustdoc-json: switch from HashMap to FxHashMap to fix non-determinism
Using `HashMap` in `rustdoc_json_types::Crate` were causing creating randomly ordered objects in the json doc files. Which might cause problems to people who are doing comparison on those files specially in CI pipelines. See https://github.com/rust-lang/rust/issues/103785#issuecomment-1307425590
This PR fixes that issue and extends the coverage of `tests/run-make/rustdoc-verify-output-files` testing ability.