Execute dotnet restore on all projects in standalone

Previously, we only did this for the solution files in the repository, but
there might be projects that are not added to any solution. This change
improves the percentage of found assemblies on the Powershell repo from
81% to 95%.

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

@@ -55,7 +55,7 @@ namespace Semmle.BuildAnalyser
             this.progressMonitor.FindingFiles(options.SrcDir);
 
             this.allSources = GetFiles("*.cs").ToArray();
-
+            var allProjects = GetFiles("*.csproj");
             var solutions = options.SolutionFile is not null
                 ? new[] { options.SolutionFile }
                 : GetFiles("*.sln");
@@ -94,6 +94,7 @@ namespace Semmle.BuildAnalyser
                 using (new FileRenamer(sourceDir.GetFiles("global.json", SearchOption.AllDirectories)))
                 {
                     Restore(solutions);
+                    Restore(allProjects);
                 }
             }