Switched .NET Standard 2.0 check to use accessibility

Author: Sergio0694

Microsoft.Toolkit.Mvvm.SourceGenerators/Attributes/NullabilityAttributesGenerator.cs

@@ -36,10 +36,9 @@ private void AddSourceCodeIfTypeIsNotPresent(GeneratorExecutionContext context,
         {
             // Check that the target attributes are not available in the consuming project. To ensure that
             // this works fine both in .NET (Core) and .NET Standard implementations, we also need to check
-            // that the target types are defined in the reference assemblies for all target runtimes. This
-            // avoids issues on .NET Standard with Roslyn also seeing internal types from referenced assemblies.
-            if (context.Compilation.GetTypeByMetadataName(typeFullName) is
-                { ContainingModule: { MetadataName: "netstandard.dll" or "System.Runtime.dll" } })
+            // that the target types are declared as public (we assume that in this case those types are from the BCL).
+            // This avoids issues on .NET Standard with Roslyn also seeing internal types from referenced assemblies.
+            if (context.Compilation.GetTypeByMetadataName(typeFullName) is { DeclaredAccessibility: Accessibility.Public })
             {
                 return;
             }