Which of the options in BuildSettings allows you to close the import scope?

I am preparing my own library that will allow me to work with UIKit in the SwiftUI style. And I'll be using my UIViewRepresentable subclass to display the preview while writing the UIKit code. Due to the fact that the names of my classes are the same as the names from SwiftUI (for example, the Text component is named exactly the same), I have name conflicts. On my working, configured project, everything works great. I just close the import scope like this:

#if DEBUG
import SwiftUI
struct MyViewController_Previews: PreviewProvider {
     static var previews: some View {
         UIViewControllerPreview {
             MyViewController()
         }
     }
}
#endif

But this same way doesn't work in my own project which I just created from scratch in Xcode. I just see classes from SwiftUI throughout the file. I guess that in the project from my work some flag is set in the Build Settings, which allows the compiler to understand scopes #if DEBUG Could you suggest which flag can I achieve this result?

This screenshot from my own project. Here i can use SwiftUI class everywhere in file.

But this is my work project. Here i can't use SwiftUI classes everywhere. Here is import SwiftUI limited to scope of #if DEBUG