Promote package suggestions to a first-class concept on IndexedDocsProviders (#16177)

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

crates/extension/src/extension_indexed_docs_provider.rs

@@ -30,6 +30,10 @@ impl IndexedDocsProvider for ExtensionIndexedDocsProvider {
         database_path
     }
 
+    async fn suggest_packages(&self) -> Result<Vec<PackageName>> {
+        Ok(Vec::new())
+    }
+
     async fn index(&self, package: PackageName, database: Arc<IndexedDocsDatabase>) -> Result<()> {
         self.extension
             .call({