import all shipped nuget packages as built-in

also would now throw if version gets changed

NuGet/Converters/ManifestPackageKeyJsonConverter.cs

@@ -8,7 +8,7 @@ public class ManifestPackageKeyJsonConverter : JsonConverter<ManifestPackageKey>
 {
     public override ManifestPackageKey Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
     {
-        if (reader.TokenType != JsonTokenType.String)
+        if (reader.TokenType is not (JsonTokenType.String or JsonTokenType.PropertyName))
             throw new JsonException("Invalid package key string");
         
         return ManifestPackageKey.Parse(reader.GetString()!);