C#: Address review comments

Author: hvitved

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

@@ -21,7 +21,7 @@ public static IEnumerable<string> CoreRuntimes
         {
             get
             {
-                var dotnetPath = FileUtils.FindExecutableOnPath("dotnet");
+                var dotnetPath = FileUtils.FindExecutableOnPath(Win32.IsWindows() ? "dotnet.exe" : "dotnet");
                 var dotnetDirs = dotnetPath != null
                     ? new[] { dotnetPath }
                     : new[] { "/usr/share/dotnet", @"C:\Program Files\dotnet" };
@@ -41,7 +41,7 @@ public static IEnumerable<string> DesktopRuntimes
         {
             get
             {
-                var monoPath = FileUtils.FindExecutableOnPath("mono");
+                var monoPath = FileUtils.FindExecutableOnPath(Win32.IsWindows() ? "mono.exe" : "mono");
                 var monoDirs = monoPath != null
                     ? new[] { monoPath }
                     : new[] { "/usr/lib/mono", @"C:\Program Files\Mono\lib\mono" };

csharp/extractor/Semmle.Util/FileUtils.cs

@@ -62,11 +62,17 @@ public static void TryDelete(string file)
         /// </summary>
         public static string FindExecutableOnPath(string exe)
         {
-            var isWindows = Win32.IsWindows();
-            var paths = Environment.GetEnvironmentVariable("PATH").Split(isWindows ? ';' : ':');
-            var exes = isWindows
-                ? Environment.GetEnvironmentVariable("PATHEXT").Split(';').Select(ext => exe + ext)
-                : new[] { exe };
+            var paths = Environment.GetEnvironmentVariable("PATH").Split(Path.PathSeparator);
+            string[] exes;
+            if (Win32.IsWindows())
+            {
+                var extensions = Environment.GetEnvironmentVariable("PATHEXT").Split(';').ToArray();
+                exes = extensions.Any(exe.EndsWith) ? new[] { exe } : extensions.Select(ext => exe + ext).ToArray();
+            }
+            else
+            {
+                exes = new[] { exe };
+            }
             var candidates = paths.Where(path => exes.Any(exe0 => File.Exists(Path.Combine(path, exe0))));
             return candidates.FirstOrDefault();
         }