feat: emit a warning when both package.publish and --index are specified

[motorailgun]

Partially adresses #16231.

What does this PR try to resolve?

cargo publish will fail, if --registry is passed and that index isn't included in package.publish in Cargo.toml. However, as described in linked issue, --index bypasses that check and may cause unexpected publication of packages.

This PR implements warning that is shown when --index and package.publish is set at the same time.

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