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/indexed_docs/src/store.rs

@@ -39,6 +39,13 @@ pub trait IndexedDocsProvider {
     /// Returns the path to the database for this provider.
     fn database_path(&self) -> PathBuf;
 
+    /// Returns a list of packages as suggestions to be included in the search
+    /// results.
+    ///
+    /// This can be used to provide completions for known packages (e.g., from the
+    /// local project or a registry) before a package has been indexed.
+    async fn suggest_packages(&self) -> Result<Vec<PackageName>>;
+
     /// Indexes the package with the given name.
     async fn index(&self, package: PackageName, database: Arc<IndexedDocsDatabase>) -> Result<()>;
 }
@@ -122,6 +129,12 @@ impl IndexedDocsStore {
             .await
     }
 
+    pub fn suggest_packages(self: Arc<Self>) -> Task<Result<Vec<PackageName>>> {
+        let this = self.clone();
+        self.executor
+            .spawn(async move { this.provider.suggest_packages().await })
+    }
+
     pub fn index(
         self: Arc<Self>,
         package: PackageName,