Skip type family defaults with hs-boot and hsig files

The top commit describes the change in detail. The previous commit was included because it makes writing test easier, but if !1767 (closed) proves controversial for some reason they can be rewritten with Cabal.

I am now more enthused with the change that when I wrote the commit message, because as @ekmett DefaultSignatures allows item-by-item overriding/defaulting whereas there's no way to use part of a deriving via instance.

WIP because I should reword the message at least, or we might want to plan a new syntax / choose opting into vs opting out of default associated types.

Sort-of fixes #17190 (closed), implements proposal https://github.com/ghc-proposals/ghc-proposals/pull/320