Fix FileSystemProvider validation during rename

Because FileSystemProvider allows specifying a file as the library and a different file name as the destination (i.e. rename scenario), it was failing during goal state validation.  However, in the FSP case, this is always valid - when a single file is specified, it can be mapped to one output file of any name.

Author: jimmylewis

src/LibraryManager/Helpers/Extensions.cs

@@ -8,6 +8,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Web.LibraryManager.Contracts;
+using Microsoft.Web.LibraryManager.Providers.FileSystem;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 
@@ -108,6 +109,16 @@ public static IReadOnlyList<string> GetInvalidFiles(this ILibrary library, IRead
 
             var invalidFiles = new List<string>();
 
+            if (library.ProviderId == FileSystemProvider.IdText
+                && library.Files.Count == 1)
+            {
+                // Ideally this would be in the FileSystemLibrary, but this is an extension method.
+                // For FileSystem provider, if a single file is specified, it is always valid.
+                // It can be mapped to a different file name at the destination.
+                // This is valid for FileSystem but not for any other provider.
+                return invalidFiles;
+            }
+
             if (files == null || !files.Any())
             {
                 return invalidFiles;
@@ -172,5 +183,15 @@ public static string GetJObjectMemberStringValue(this JObject jObject, string pr
             return propertyValue;
         }
 
+#if NETFRAMEWORK
+        /// <summary>
+        /// Destructs the KeyValuePair into separate values for key and value.
+        /// </summary>
+        public static void Deconstruct<T1, T2>(this KeyValuePair<T1, T2> pair, out T1 key, out T2 value)
+        {
+            key = pair.Key;
+            value = pair.Value;
+        }
+#endif
     }
 }

src/LibraryManager/Providers/FileSystem/FileSystemProvider.cs

@@ -17,6 +17,7 @@ namespace Microsoft.Web.LibraryManager.Providers.FileSystem
     /// <summary>Internal use only</summary>
     internal sealed class FileSystemProvider : BaseProvider
     {
+        public const string IdText = "filesystem";
         private FileSystemCatalog _catalog;
 
         /// <summary>Internal use only</summary>
@@ -28,7 +29,7 @@ public FileSystemProvider(IHostInteraction hostInteraction)
         /// <summary>
         /// The unique identifier of the provider.
         /// </summary>
-        public override string Id => "filesystem";
+        public override string Id => IdText;
 
         /// <summary>
         /// Hint text for the library id.