This patch tweaks the stability inheritance infrastructure so that
`#{stable]` attributes are not inherited. Doing so solves two problems:
1. It allows us to mark module *names* as stable without accidentally
marking the items they contain as stable.
2. It means that a `#[stable]` attribution must always appear directly
on the item it applies to, which makes it easier for reviewers to catch
changes to stable APIs.
Fixes #17484