indexed_docs: Remove some unnecessary cloning (#30236)

Other small patch to reduce allocations.

`.iter().cloned().collect()` calls `Clone` per element, whereas
`.into_iter().collect()` preallocates the `Vec`.

The Zed repo for example has up to 1700 packages on some build
configurations, meaning this change theoretically saves up to 1699
allocations. It's likely the compiler has already optimized this away,
but it's good to be explicit.

Release Notes:

- N/A

crates/indexed_docs/src/providers/rustdoc.rs

@@ -79,7 +79,7 @@ impl IndexedDocsProvider for LocalRustdocProvider {
 
         *WORKSPACE_CRATES.write() = Some((workspace_crates.clone(), Instant::now()));
 
-        Ok(workspace_crates.iter().cloned().collect())
+        Ok(workspace_crates.into_iter().collect())
     }
 
     async fn index(&self, package: PackageName, database: Arc<IndexedDocsDatabase>) -> Result<()> {