privacy: Fix privacy lints in RPITITs

[petrochenkov]

Visit RPITITs and report private_interfaces, private_bounds and exported_private_dependencies in them (these are regular, non-deprecation lints).
New hard errors are not reported, #146470 is for hard errors.
So this PR doesn't contain any breakage or language changes.

[rustbot]

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[rustbot]

This PR was rebased onto a different main 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.