socket.io externs: promote export = const name:T to static class T

socket.io has this pattern
```typescript
declare const SocketIO: SocketIOStatic;
export = SocketIO;

interface SocketIOStatic {
}
```

The interface `SocketIOStatic` shouldn't be generated (but is) and the `export = SocketIO;` should promote the variable `SocketIO` into a class of static fields