This PR promotes package suggestions to a first-class concept on the
`IndexedDocsProvider` trait.
This will allow any implementer of `IndexedDocsProvider` to provide a
list of package names to suggest for use with `/docs`.
For the docs.rs provider we use the 250 most popular Rust crates (as
identified [here](https://lib.rs/std)), and for the rustdoc provider we
use the packages in the Cargo workspace.
Release Notes:
- N/A
