Flatten out the NSE file structure and switch room fetching to the notificationClient instead of the base one

- the notification client uses its internal in-memory store backed client which has up to date data after running the sliding sync loop
- see https://github.com/matrix-org/matrix-rust-sdk/pull/4991 for more details

NSE/Sources/NSEUserSession.swift

@@ -76,7 +76,7 @@ final class NSEUserSession {
     
     func roomForIdentifier(_ roomID: String) -> Room? {
         do {
-            return try baseClient.getRoom(roomId: roomID)
+            return try notificationClient.getRoom(roomId: roomID)
         } catch {
             MXLog.error("Failed retrieving room with error: \(error)")
             return nil