Limit registry-index dependency field to registry sources only

[moabo3li]

registry-index is an internal-only field that Cargo uses when packaging crates. It should not appear in user-written Cargo.toml files as it allows bypassing the documented pattern of using registry = "name" with .cargo/config.toml.

Fixes #15503

[rustbot]

r? @weihanglo

rustbot has assigned @weihanglo. They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[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.

[rustbot]

:umbrella: The latest upstream changes (possibly c46423de7351e3c4c734b2faa86088a9f5d1302b) made this pull request unmergeable. Please resolve the merge conflicts.