Avoid documenting top-level private imports
PR #88447 aimed to make rustdoc's `--document-private-items` mode only document imports that are visible outside the importing module. Unfortunately, I inadvertently set things up so that imports at the crate top-level are always documented, regardless of their visibility. This behavior was unintended and is [not desirable](https://github.com/rust-lang/rust/issues/90865#issuecomment-971172649).
This PR treats top-level imports as never being visible outside their parent module. In practice, the only way a top-level import can be visible externally is if it's fully public, and there's a seperate check for that.
It's worth calling attention to the fact that this change means that `pub(crate)` imports will be visible in lower level modules, but not at the top-level. This is because, at the top level of the crate, `pub(crate)` means the same thing as `pub(self)`.
It turned out that there were existing tests checking for the only behavior, which I didn't notice at the time of my previous PR. I have updated them to check for the new behavior and substantially extended them to handle differences between the top-level module and lower level modules. I may have gone overboard, so please tell me if there's anything I should cut.
r? `@jyn514`
Fixes#90865.
2e055d92e0