Finer grained determination of visibility for extension methods

[mknejp]

Currently the generated static class containing extension methods is either public or internal based on the visibility of the variant itself, but it doesn't take into consideration the visibility of the value types.

In addition to that, now that we support generic types we can end up in a scenario like the following:

public partial class Variant<T1> { /* ... */ }

internal partial class Variant<T2, T2> { /* ... */ }

// generated code:
internal static class VariantEx
{
  // Extension methods for Variant<T1> are public
  ...
  // Extension methods for Variant<T1, T2> are public
  ...
}

Even though this is a very questionable design, users can write code like that and it should not generate a broken implementation.

To support this properly we need to determine the visibility for the extension class itself separately from the visibility of the generated extension methods. So in the above example VariantEx should be public with the extension methods for Variant<T1> generated public and for Variant<T1, T2> generated internal.