Add VRChat and GitHub packages to Unity project

Packages/com.vrchat.vrcsdk3/Editor/UPMImporter/UPMImporter.cs

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+using System.IO;
+using System.Linq;
+using UnityEditor;
+using UnityEditor.PackageManager;
+using UnityEditor.PackageManager.Requests;
+using UnityEngine;
+
+namespace VRC.Udon.Editor {
+    [InitializeOnLoad]
+    public class UPMImporter
+    {
+        // Add packages here to auto-import
+        public static string[] requiredPackages =
+        {
+            "com.unity.cinemachine@2.6.3",
+            "com.unity.postprocessing@2.3.0",
+            "com.unity.textmeshpro@1.4.1",
+        };
+        
+        private static ListRequest list;
+        
+        static UPMImporter()
+        {
+            list = Client.List();
+            EditorApplication.update += Update;
+        }
+
+        public static void Update()
+        {
+            // Exit early if we're still gathering the list
+            if (!list.IsCompleted) return;
+            
+            // Unsubscribe from Update once the list is ready
+            EditorApplication.update -= Update;
+
+            var localPackages = list.Result;
+            bool importedNewPackage = false;
+            foreach (string packageName in requiredPackages)
+            {
+                var packageNameNoVersion = packageName.Substring(0, packageName.IndexOf('@'));
+                if(!localPackages.Any(p => p.name == packageNameNoVersion))
+                {
+                    Install(packageName);
+                    importedNewPackage = true;
+                }
+            }
+            
+            // if Unity tried to import SDK3 before required packages, it will have old errors showing.
+            if(importedNewPackage) ClearLog();
+        }
+
+        public static bool Install(string id)
+        {
+            var request = Client.Add(id);
+            while (!request.IsCompleted) {};
+            if(request.Error != null)Debug.LogError(request.Error.message);
+            return request.Error == null;
+        }
+        
+        public static void ClearLog()
+        {
+            System.Reflection.Assembly assembly = System.Reflection.Assembly.GetAssembly(typeof(SceneView));
+
+            System.Type type = assembly.GetType("UnityEditor.LogEntries");
+            System.Reflection.MethodInfo method = type.GetMethod("Clear");
+            method.Invoke(new object(), null);
+        }
+
+    }
+
+}