Non-abstract types also have skolem nature

In #12680, we determined that abstract types from signatures need to be treated like skolem variables.

But it turns out other types from signatures also need to be treated like skolem variables too:

{-# LANGUAGE GADTs #-}
unit p where
    signature M1 where
        data A = MkA
    signature M2 where
        data A = MkA
    module A where
        import qualified M1
        import qualified M2
        f :: M1.A ~ M2.A => a -> b
        f x = x

This is bad because these As could be the same thing in the end. I think the real, final fix, is to not test for "skolem abstractness" (as we do now), but just look and see if the Name for the TyCon is a name hole or not. Then we can eliminate skolem abstract completely.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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