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

.carve/ignore

@@ -81,7 +81,7 @@ frontend.ui/_emoji-init-data
 ;; placeholder var for defonce
 frontend.worker.rtc.op-mem-layer/_sync-loop-canceler
 ;; Used by shadow.cljs
-frontend.db-worker/init
+frontend.worker.db-worker/init
 ;; WIP fn, remove when it's ready
 frontend.worker.rtc.asset-sync/<loop-for-assets-sync
 ;; Future use?