backend: extract some deps as paramters
small fixes
add some NonNull annotations for Backend
CodeManager only needs a rpcSite not a full backend
remove unused code
remove unused code in InternalBackend
simplify CodeBundle and move api to backend plugin
simplify BackendManager
make IShutdownCallback a standalone interface
remove unused BackendHelper
abstract away the backend from XrefService
abstract away the backend from OpenService
abstract away the backend from ScannerService
abstract away the backend from IndentService
abstract away the backend from SearchServerService
abstract away the backend from ContextAssistService
merge engine updates