Rollup merge of #149043 - aDotInTheVoid:is-this-real-is-this-out-of-spite-does-it-matter, r=GuillaumeGomez

 rustdoc-json: add rlib path to ExternalCrate to enable robust crate resolution

Historically, it's not been possible to robustly resolve a cross-crate item in rustdoc-json. If you had a `Id` that wasn't in `Crate::index` (because it was defined in a different crate), you could only look it up it `Crate::paths`. But there, you don't get the full information, only an `ItemSummary`. This tells you the `path` and the `crate_id`.

But knowing the `crate_id` isn't enough to be able to build/find the rustdoc-json output with this item. It's only use is to get a `ExternalCrate` (via `Crate::external_crates`). But that only tells you the `name` (as a string). This isn't enough to uniquely identify a crate, as there could be multiple versions/features [^1] [^2].

This was originally proposed to be solved via `@LukeMathWalker's` `--orchestrator-id` proposal (https://github.com/rust-lang/compiler-team/issues/635). But that requires invasive changes to cargo/rustc. This PR instead implements `@Urgau's` proposal to re-use the path to a crate's rmeta/rlib as a unique identifer. Callers can use that to determine which package it corresponds to in the language of the build-system above rustc. E.g. for cargo, `cargo rustdoc --message-format=json --output-format=json -Zunstable-options`).

(Once you've found the right external crate's rustdoc-json output, you still need to resolve the path->id in that crate. But that's """just""" a matter of walking the module tree. We should probably still make that nicer (by, for example, allowing sharing `Id`s between rustdoc-json document), but that's a future concern)

For some notes from RustWeek 2025, where this was designed, see https://hackmd.io/0jkdguobTnW7nXoGKAxfEQ

CC `@obi1kenobi` (who wants this for cargo-semver-checks [^3]), `@epage` (who's conversations on what and wasn't possible with cargo informed taking this approach to solve this problem)

r? `@GuillaumeGomez`

## TODO:

- [x] Docs: [Done](https://github.com/rust-lang/rust/compare/e4cdd0c24a994fed354081b5f907680a11f2ddc5..457ed4edb184997d5d6f879c6a220bc4d69ff6fd)
- [x] Tests: [Done](https://github.com/rust-lang/rust/compare/2e1b954dc52bf7e5a6e9311394df760db37d383f..4d00c1a7ee5e03d1e78801cc01a85dac08ab603b)

[^1]: https://github.com/rust-lang/compiler-team/issues/635#issue-1714254865 § Problem
[^2]: https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/Identifying.20external.20crates.20in.20Rustdoc.20JSON/with/352701211
[^3]: https://github.com/obi1kenobi/cargo-semver-checks/issues/638