Rollup merge of #131936 - jalil-salame:rustdoc-types-rustc-hash, r=aDotInTheVoid

feat(rustdoc-json-types): introduce rustc-hash feature

This allows the public `rustdoc-types` crate to expose this feature easily and allows consumers of the crate to get the performance advantages from doing so.

The reasoning for this was discussed on [Zulip][1]

Changes:
- Make `rustc-hash` optional but default to including it
- Rename all occurrences of `FxHashMap` to `HashMap`.
- Feature gate the import and rename the imported `FxHashMap` to `HashMap`
- Introduce a type alias `FxHashMap` which resolves to the currently used `HashMap` (`rustc_hash::FxHashMap` or `std::collections::HashMap`) for use in `src/librustdoc`.

[1]: https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/rustc-hash.20and.20performance.20of.20rustdoc-types

**extra context from the zulip thread:**

- `@obi1kenobi` requested benchmarks of the switch to `rustc-hash`
- I benchmarked switching `rustdoc-types` to `rustc-hash` which yielded a ~300ms improvement to `cargo-semver-checks`'s index building step (this step is done twice so the improvements are ~150ms per index).
- The benchmarks were presented in Zulip and people were in favor of introducing `rustc-hash` to the public `rustdoc-types` crate.
- There were differing opinions on how to introduce the dependency:
  1. "Hard" dependency: remove use of `std::collections::HashMap` in favor of `FxHashMap`.
  2. "Soft" dependency: make optional and introduce a feature then enable/disable it by default (this PR).
  3. ~~Make `rustdoc-types` generic and expose the `RandomState`~~ (a lot of work & complexity for little gain over a feature gate).

`@obi1kenobi` and I prefer the feature gate so that is what I am adding here.

My reasons for the preference are:
- `cargo-semver-checks` is especially perf sensitive, we don't expect people to care about ~150ms extra time when reading in a 500MB file (the size of the sample we used for benchmarking).
- Keeping `rustdoc-types` lean by having its only direct dependency be `serde` is nice for the general consumer of the crate.
- `rustc-hash` is not HashDOS resistant (but it is questionable whether `rustdoc-types` would be used on adversarial inputs).

r? `@aDotInTheVoid`

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