refactor: organize worker namespaces and lint them

Organize them to live under src/main/frontend/worker and add a lint to ensure
that common code with frontend is only under frontend/common/.
Add a linter to ensure that worker doesn't depend on frontend.
Motivated to fix after recent worker breakage fixed by
75463c4df4

.github/workflows/build.yml

@@ -120,6 +120,9 @@ jobs:
       - name: Lint invalid translation entries
         run: bb lang:validate-translations
 
+      - name: Lint to keep worker independent of frontend
+        run: bb lint:worker-and-frontend-separate
+
       - name: Lint to keep db and file graph code separate
         run: bb lint:db-and-file-graphs-separate