Minor fixes in standalone extraction

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp.Standalone/AssemblyCache.cs

@@ -41,6 +41,8 @@ private void AddReferenceDirectory(string dir)
             }
         }
 
+        private static readonly Version emptyVersion = new Version(0, 0, 0, 0);
+
         /// <summary>
         /// Indexes all DLLs we have located.
         /// Because this is a potentially time-consuming operation, it is put into a separate stage.
@@ -55,7 +57,9 @@ private void IndexReferences()
 
             // Index "assemblyInfo" by version string
             // The OrderBy is used to ensure that we by default select the highest version number.
-            foreach (var info in assemblyInfoByFileName.Values.OrderBy(info => info.Id))
+            foreach (var info in assemblyInfoByFileName.Values
+                .OrderBy(info => info.Name)
+                .ThenBy(info => info.Version ?? emptyVersion))
             {
                 foreach (var index in info.IndexStrings)
                     assemblyInfoById[index] = info;

csharp/extractor/Semmle.Extraction.CSharp/Extractor/CsProjFile.cs

@@ -100,7 +100,7 @@ private static (string[] csFiles, string[] references, string[] projectReference
 
             // Figure out if it's dotnet core
 
-            var netCoreProjectFile = root.GetAttribute("Sdk") == "Microsoft.NET.Sdk";
+            var netCoreProjectFile = root.GetAttribute("Sdk").StartsWith("Microsoft.NET.Sdk");
 
             if (netCoreProjectFile)
             {