import all shipped nuget packages as built-in

also would now throw if version gets changed

NuGet/Converters/RuntimeFrameworkJsonConverter.cs

@@ -0,0 +1,29 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using NuGet.Models;
+
+namespace NuGet.Converters;
+
+public class RuntimeFrameworkJsonConverter : JsonConverter<NuGetRuntimeFramework>
+{
+    public override NuGetRuntimeFramework Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType is not (JsonTokenType.String or JsonTokenType.PropertyName))
+            throw new JsonException("Invalid runtime framework string");
+        
+        return NuGetRuntimeFramework.Parse(reader.GetString()!);
+    }
+
+    public override void Write(Utf8JsonWriter writer, NuGetRuntimeFramework value, JsonSerializerOptions options)
+    {
+        writer.WriteStringValue(value.ToString());
+    }
+
+    public override NuGetRuntimeFramework ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert,
+        JsonSerializerOptions options) => Read(ref reader, typeToConvert, options);
+
+    public override void WriteAsPropertyName(Utf8JsonWriter writer, NuGetRuntimeFramework value, JsonSerializerOptions options)
+    {
+        writer.WritePropertyName(value.ToString());
+    }
+}