chore: reference the Netcode for Entities assemblies by GUID

Three asmdefs named "Unity.NetCode" and "Unity.NetCode.Editor" as strings. Unity silently drops an
asmdef reference it cannot resolve, so renaming either assembly would make every UNIFIED_NETCODE code
path fail to compile with nothing pointing at the cause. A GUID reference resolves to whichever
asmdef asset carries that GUID regardless of the name inside it, which is how NGO's own editor
assembly rename stayed transparent to projects referencing it.

The GUIDs are read from the installed package's .asmdef.meta files, not derived:
  Unity.NetCode         953adc2a6b8b4e3c8df5b728bcd546e9
  Unity.NetCode.Editor  eb8cf780bf058694ebf7ec5cf5b8cfa3

Unity.Entities is left by name. It is not the assembly at risk of a rename, and converting it would
add surface that cannot be checked here for no benefit.

Not verified locally: confirming a GUID reference resolves needs an editor with both packages
installed. Worth an import in a project that has Netcode for Entities before this is relied on - if a
GUID is wrong the reference drops silently, which is the failure this is meant to prevent.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/Editor/Unity.Netcode.Editor.asmdef

@@ -8,8 +8,8 @@
         "Unity.Networking.Transport",
         "Unity.Services.Core",
         "Unity.Services.Authentication",
-        "Unity.NetCode",
-        "Unity.NetCode.Editor"
+        "GUID:953adc2a6b8b4e3c8df5b728bcd546e9",
+        "GUID:eb8cf780bf058694ebf7ec5cf5b8cfa3"
     ],
     "includePlatforms": [
         "Editor"

com.unity.netcode.gameobjects/Runtime/Unity.Netcode.Runtime.asmdef

@@ -14,7 +14,7 @@
         "Unity.Collections",
         "Unity.Burst",
         "Unity.Mathematics",
-        "Unity.NetCode",
+        "GUID:953adc2a6b8b4e3c8df5b728bcd546e9",
         "Unity.Entities"
     ],
     "includePlatforms": [],

com.unity.netcode.gameobjects/Tests/Runtime/Unity.Netcode.Runtime.Tests.asmdef

@@ -14,7 +14,7 @@
         "Unity.Mathematics",
         "UnityEngine.TestRunner",
         "UnityEditor.TestRunner",
-        "Unity.NetCode",
+        "GUID:953adc2a6b8b4e3c8df5b728bcd546e9",
         "Unity.Entities"
     ],
     "includePlatforms": [],