Extract ExtensionIndexedDocsProvider to indexed_docs crate (#20607)

This PR extracts the `ExtensionIndexedDocsProvider` implementation to
the `indexed_docs` crate.

To achieve this, we introduce a new `Extension` trait that provides an
abstracted interface for calling an extension. This trait resides in the
`extension` crate, which has minimal dependencies and can be depended on
by other crates, like `indexed_docs`.

We're then able to implement the `ExtensionIndexedDocsProvider` without
having any knowledge of the Wasm-specific internals of the extension
system.

Release Notes:

- N/A

crates/indexed_docs/src/extension_indexed_docs_provider.rs

@@ -0,0 +1,53 @@
+use std::path::PathBuf;
+use std::sync::Arc;
+
+use anyhow::Result;
+use async_trait::async_trait;
+use extension::Extension;
+
+use crate::{IndexedDocsDatabase, IndexedDocsProvider, PackageName, ProviderId};
+
+pub struct ExtensionIndexedDocsProvider {
+    extension: Arc<dyn Extension>,
+    id: ProviderId,
+}
+
+impl ExtensionIndexedDocsProvider {
+    pub fn new(extension: Arc<dyn Extension>, id: ProviderId) -> Self {
+        Self { extension, id }
+    }
+}
+
+#[async_trait]
+impl IndexedDocsProvider for ExtensionIndexedDocsProvider {
+    fn id(&self) -> ProviderId {
+        self.id.clone()
+    }
+
+    fn database_path(&self) -> PathBuf {
+        let mut database_path = PathBuf::from(self.extension.work_dir().as_ref());
+        database_path.push(self.extension.manifest().id.as_ref());
+        database_path.push("docs");
+        database_path.push(format!("{}.0.mdb", self.id));
+
+        database_path
+    }
+
+    async fn suggest_packages(&self) -> Result<Vec<PackageName>> {
+        let packages = self
+            .extension
+            .suggest_docs_packages(self.id.0.clone())
+            .await?;
+
+        Ok(packages
+            .into_iter()
+            .map(|package| PackageName::from(package.as_str()))
+            .collect())
+    }
+
+    async fn index(&self, package: PackageName, database: Arc<IndexedDocsDatabase>) -> Result<()> {
+        self.extension
+            .index_docs(self.id.0.clone(), package.as_ref().into(), database)
+            .await
+    }
+}