C#: Address more review comments.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

@@ -663,11 +663,11 @@ public static bool ShouldExtractSymbol(this ISymbol symbol)
             }
             if (symbol is IMethodSymbol method)
             {
-                return method.ExplicitInterfaceImplementations.Any(m => m.ContainingType.DeclaredAccessibility == Accessibility.Public);
+                return method.ExplicitInterfaceImplementations.Any(m => m.ContainingType.ShouldExtractSymbol());
             }
             if (symbol is IPropertySymbol property)
             {
-                return property.ExplicitInterfaceImplementations.Any(m => m.ContainingType.DeclaredAccessibility == Accessibility.Public);
+                return property.ExplicitInterfaceImplementations.Any(m => m.ContainingType.ShouldExtractSymbol());
             }
             return false;
         }