Assorted cleanups of stable runtime crates (#3205)

## Motivation and Context
Stable crates MUST only expose other stable crates.

## Description
 This:
- fixes the remaining issues
- adds a lint tool to be sure we don't expose unstable crates by
accident in the future

## Testing
CI run

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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