Update SCUI_DEFAULT_BACKEND handling in Package.swift to allow it to override non-desktop backends

[stackotter]

At the moment SCUI_DEFAULT_BACKEND is only respected when targeting desktop platforms, because on other platforms we have to use SwiftPM conditionals. To allow SCUI_DEFAULT_BACKEND to override the backend used when building for a non-desktop platform, we should update our Package.swift logic to unconditionally use defaultBackend when it gets set by the user.

Should be a simple fix, just wanted to keep it separate from my current changes cause it feels possible to mess up.