Nested `private` custom formatters result in CS0122: inaccessible from source generated resolver

Given code like this:

```cs
class A {
   private class JTokenFormatter : IMessagePackFormatter<JToken?> { /* ... */ }
}
```

The source generated resolver will try to access that formatter, and cannot due to visibility rules. The compilation breaks with a CS0122 error.

We should skip private formatters, and possibly report a warning diagnostic.