This hack did not work in fact

Author: mxcl

Package.swift

@@ -9,11 +9,6 @@ let pkg = Package(
     targets: [
         .target(name: "Path", path: "Sources"),
         .testTarget(name: "PathTests", dependencies: ["Path"]),
-    ]
+    ],
+    swiftLanguageVersions: [.v4, .v4_2]
 )
-
-pkg.swiftLanguageVersions = [.v4_2]
-
-#if swift(>=5)
-pkg.swiftLanguageVersions.append(.v5)
-#endif

Package@swift-5.0.swift

@@ -0,0 +1,14 @@
+// swift-tools-version:5.0
+import PackageDescription
+
+let pkg = Package(
+    name: "Path.swift",
+    products: [
+        .library(name: "Path", targets: ["Path"]),
+    ],
+    targets: [
+        .target(name: "Path", path: "Sources"),
+        .testTarget(name: "PathTests", dependencies: ["Path"]),
+    ],
+    swiftLanguageVersions: [.v4, .v4_2, .v5]
+)