Embed package into your Unity project to modify the code more easily :)

EmbedPackage.cs

using System.IO;
using UnityEditor.PackageManager;
using UnityEngine;

namespace UnityEditor.Extensions
{
    #if UNITY_2017_3_OR_NEWER
    
    /// <summary>
    /// Editor extension for embedding packages as a local copy in the project.
    /// This can be useful in case you want to modify the package's source code.
    /// </summary>
    public static class EmbedPackage
    {
        [MenuItem("Assets/Embed Package", false, 1000000)]
        private static void EmbedPackageMenuItem()
        {
            var selection = Selection.activeObject;
            var packageName = Path.GetFileName(AssetDatabase.GetAssetPath(selection));

            Debug.Log($"Embedding package '{packageName}' into the project.");

            Client.Embed(packageName);

            AssetDatabase.Refresh();
        }

        [MenuItem("Assets/Embed Package", true)]
        private static bool EmbedPackageValidation()
        {
            var selection = Selection.activeObject;

            if (selection == null)
            {
                return false;
            }

            var path = AssetDatabase.GetAssetPath(selection);
            var folder = Path.GetDirectoryName(path);
            
            // We only deal with direct folders under Packages/
            return folder == "Packages";
        }
    }
    
    #endif
}

I was having an issue where Selection.activeObject was not reflecting the folder I clicked. Perhaps Unity changed their interface to make folders no longer behave as a selected object. Here is my fix:

using UnityEditor.PackageManager;
using UnityEngine;

namespace UnityEditor.Extensions
{
   #if UNITY_2017_3_OR_NEWER

   /// <summary>
   /// Editor extension for embedding packages as a local copy in the project.
   /// This can be useful in case you want to modify the package's source code.
   /// </summary>
   public static class EmbedPackage
   {
      [MenuItem("Assets/Embed Package", false, 1000000)]
      private static void EmbedPackageMenuItem()
      {
         var packageName = Path.GetFileName(GetSelectionPath());

         Debug.Log($"Embedding package '{packageName}' into the project.");

         Client.Embed(packageName);

         AssetDatabase.Refresh();
      }

      [MenuItem("Assets/Embed Package", true)]
      private static bool EmbedPackageValidation()
      {
         var path = GetSelectionPath();
         var folder = Path.GetDirectoryName(path);

         // We only deal with direct folders under Packages/
         return folder == "Packages";
      }

      private static string GetSelectionPath()
      {
         if (Selection.assetGUIDs.Length == 0)
            return "";

         string clickedAssetGuid = Selection.assetGUIDs[0];
         string clickedPath = AssetDatabase.GUIDToAssetPath(clickedAssetGuid);
         return clickedPath;
      }
   }

   #endif
}