added checking for build target to throw an error if no plugin present

Author: ritamerkl

Packages/com.unity.inputsystem/InputSystem/Editor/InputSystemPluginControl.cs

@@ -0,0 +1,60 @@
+using System.Linq;
+using NUnit.Framework;
+using UnityEditor;
+
+namespace UnityEngine.InputSystem.Editor
+{
+    public class InputSystemPluginControl
+    {
+        [InitializeOnLoadMethod]
+        private static void CheckForExtension()
+        {
+            ThrowWarningOnMissingPlugin();
+        }
+
+        private static readonly BuildTarget[] targetNoPluginNeeded = new BuildTarget[]
+        {
+            BuildTarget.StandaloneOSX,
+            BuildTarget.StandaloneWindows,
+            BuildTarget.iOS,
+            BuildTarget.Android,
+            BuildTarget.StandaloneWindows64,
+            BuildTarget.WebGL,
+            BuildTarget.WSAPlayer,
+            BuildTarget.StandaloneLinux64,
+            BuildTarget.tvOS,
+            BuildTarget.LinuxHeadlessSimulation,
+            BuildTarget.PS5,
+            BuildTarget.EmbeddedLinux,
+            BuildTarget.QNX,
+            BuildTarget.VisionOS,
+            BuildTarget.ReservedCFE,
+            BuildTarget.NoTarget
+        };
+
+        static bool BuildTargetNeedsPlugin()
+        {
+            BuildTarget target = EditorUserBuildSettings.activeBuildTarget;
+            return !targetNoPluginNeeded.Contains(target);
+        }
+
+        private static string plugInName = "com.unity.inputsystem.";
+        private static bool IsPluginInstalled()
+        {
+            var registeredPackages = UnityEditor.PackageManager.PackageInfo.GetAllRegisteredPackages();
+            foreach (var package in registeredPackages)
+            {
+                if (package.name.StartsWith(plugInName)) //TODO better ??
+                    return true;
+            }
+            return false;
+        }
+
+        private static void ThrowWarningOnMissingPlugin()
+        {
+            if (BuildTargetNeedsPlugin() && !IsPluginInstalled())
+                Debug.Log("No plugin installed!");
+            // TODO Assert.
+        }
+    }
+}