SI-8793 Fix patmat regression with extractors, existentials

In the same vein as SI-8128 / 3e9e2c65a, revert to the 2.10.x style
of determining the types of the product elements of an extractor
when using `TupleN`.

I believe we can discard the special casing for Option/Tuple/Seq
altogether with judicious application of `repackExistential` in
`unapplyMethodTypes`. That ought to also fix fix SI-8149. But I'll
target that work at 2.12.x.