Provide a way to search for database items by name and language

We'll use this to check whether a database for our ql pack already exists.

While there are other methods that search for a database item by URI, we
only have a language chosen by the user and an nwo ("github/codeql").

So let's introduce a way to search for the db based on the information we
have.

extensions/ql-vscode/src/local-databases.ts

@@ -897,6 +897,20 @@ export class DatabaseManager extends DisposableObject {
     }
   }
 
+  public async digForDatabaseItem(
+    language: string,
+    databaseNwo: string,
+  ): Promise<DatabaseItem | undefined> {
+    const dbItems = this.databaseItems || [];
+    const dbs = dbItems.filter(
+      (db) => db.language === language && db.name === databaseNwo,
+    );
+    if (dbs.length === 0) {
+      return undefined;
+    }
+    return dbs[0];
+  }
+
   /**
    * Returns the index of the workspace folder that corresponds to the source archive of `item`
    * if there is one, and -1 otherwise.

extensions/ql-vscode/test/vscode-tests/minimal-workspace/local-databases.test.ts

@@ -785,6 +785,39 @@ describe("local databases", () => {
     });
   });
 
+  describe("digForDatabaseItem", () => {
+    describe("when the item exists", () => {
+      it("should return the database item", async () => {
+        const mockDbItem = createMockDB();
+
+        await (databaseManager as any).addDatabaseItem(
+          {} as ProgressCallback,
+          {} as CancellationToken,
+          mockDbItem,
+        );
+
+        const databaseItem = await databaseManager.digForDatabaseItem(
+          mockDbItem.language,
+          mockDbItem.name,
+        );
+
+        expect(databaseItem!.language).toEqual(mockDbItem.language);
+        expect(databaseItem!.name).toEqual(mockDbItem.name);
+      });
+    });
+
+    describe("when the item doesn't exist", () => {
+      it("should return nothing", async () => {
+        const databaseItem = await databaseManager.digForDatabaseItem(
+          "ruby",
+          "mock-database-name",
+        );
+
+        expect(databaseItem).toBeUndefined();
+      });
+    });
+  });
+
   function createMockDB(
     mockDbOptions = MOCK_DB_OPTIONS,
     // source archive location must be a real(-ish) location since