How to load SKScene from a swift package?

Sounds plausible. Try this (untested):

extension SKNode {
    convenience init?(data: Data) {
        guard let archive = try? NSKeyedUnarchiver(forReadingFrom: data) else { return nil }
        self.init(coder: archive)
    }
    convenience init?(url: URL) {
        guard let data = try? Data(contentsOf: url) else { return nil }
        self.init(data: data)
    }
    convenience init?(_ name: String, bundle: Bundle) {
        guard let url = bundle.url(forResource: name, withExtension: nil) else { return nil }
        self.init(url: url)
    }
}