Emit warning when building package manifest

We also have a check in the plugin code but SwiftPM 5.9.1 and earlier
try to build some completely unsupported packages like swift-nio and
fail with a cryptic error message. Emitting a warning early on should
help users understand what's going on.

Package.swift

@@ -3,6 +3,10 @@
 
 import PackageDescription
 
+#if swift(<5.9.2)
+#warning("Swift 5.9.1 or earlier is not supported by carton")
+#endif
+
 let package = Package(
   name: "carton",
   platforms: [.macOS(.v13)],