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

[aDotInTheVoid]

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 (rust-lang/compiler-team#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 Ids 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:

• Docs
• Tests

Footnotes

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 ↩

[rustbot]

rustdoc-json-types is a public (although nightly-only) API. If possible, consider changing src/librustdoc/json/conversions.rs; otherwise, make sure you bump the FORMAT_VERSION constant.

cc @CraftSpider, @aDotInTheVoid, @Enselic, @obi1kenobi

[rustbot]

⚠️ Warning ⚠️

• There are username mentions (such as @user) in the commit messages of the following commits.
  Please remove the mentions to avoid spamming these users.
  • c7f4e67