Swift: Update to package manager 5.1

swift/Sources/GildedRose/GildedRose.swift

@@ -2,7 +2,7 @@
 public class GildedRose {
     var items:[Item]
     
-    required public init(items:[Item]) {
+    public init(items:[Item]) {
         self.items = items
     }
     

swift/Sources/GildedRoseApp/main.swift

@@ -1,3 +1,4 @@
+import GildedRose
 
 let items = [
     Item(name: "+5 Dexterity Vest", sellIn: 10, quality: 20),