feat: rebuild(-ish) rustdoc json for different version of same crates

[motorailgun]

Fix #16291.

What does this PR try to resolve?

As discussed in the linked issue, cargo rustdoc --output-format=json doesn't rebuild for same crate name but with different versions. This is because current cargo implementation around rustdoc only specifies output paths/names by only its crate names, but not with version(hash) info.

This PR forces new layout for json output, and uplift the file into backward-compatible place.

How to test and review this PR?

• JSON file's path could just be doc/<crate-name>-<hash>.json as this option is still unstable.
• Uplifting will always happen even if uplifted json is fresh, and/or new layout is manually specified. (could not figure out how to prevent)

[rustbot]

r? @epage

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The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
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[motorailgun]

Updated to use new directory layout.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.