C#: Make sure System.Private.CoreLib is added only once as a reference in standalone extraction

Author: tamasvajk

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

@@ -100,6 +100,18 @@ public BuildAnalysis(Options options, IProgressMonitor progress)
                 dllDirNames.Add(runtimeLocation);
             }
 
+            if (options.UseMscorlib)
+            {
+                // Add mscorlib.dll or System.Private.CoreLib.dll to the list of DLLs to reference.
+                var loc = typeof(object).Assembly.Location;
+                var dir = Path.GetDirectoryName(loc);
+                if (dir != null)
+                {
+                    progressMonitor.Log(Util.Logging.Severity.Debug, $"Adding folder {dir} to DLL search path.");
+                    dllDirNames.Add(dir);
+                }
+            }
+
             // These files can sometimes prevent `dotnet restore` from working correctly.
             using (new FileRenamer(sourceDir.GetFiles("global.json", SearchOption.AllDirectories)))
             using (new FileRenamer(sourceDir.GetFiles("Directory.Build.props", SearchOption.AllDirectories)))
@@ -122,11 +134,6 @@ public BuildAnalysis(Options options, IProgressMonitor progress)
 
             ResolveConflicts();
 
-            if (options.UseMscorlib)
-            {
-                UseReference(typeof(object).Assembly.Location);
-            }
-
             // Output the findings
             foreach (var r in usedReferences.Keys)
             {