fix #15222 recursively check for product ctor accessibility

[bishabosha]

mostly a refactoring, except now whyNotGenericSum will benefit by checking the constructor accessibility of child product types.

Removes the declScope parameter from whyNotGenericSum because it is not useful. In all cases it is populated by either the companion object of self when it is valid to use as the mirror, or it is the context owner when we should use an anonymous mirror. This is awkward because we end up calling useCompanionAsSumMirror twice in each recursive call.

Also fixes an inconsistency where scala.Nil.type would not get a singleton mirror, causing a runtime error (and extends it to aliases of singleton enum values)

fixes #15222
fixes #15234

[odersky]

Otherwise LGTM