Rustdoc-Json: Don't remove impls for items imported from private modules
After #99287, items in private modules may still be in the json output, if a public import accesses them. To reflect this, items that are imported need to be marked as retained in the `Stripper` pass, so their impls arn't removed by `ImplStripper`.
[More context on zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Populating.20cache.2Eimpls), thanks to @ jyn514 for helping debug this.
``@rustbot`` modify labels: +A-rustdoc-json +T-rustdoc
r? ``@GuillaumeGomez``
Fixes#100252Fixes#100242