Otherwise extra packages in scope can't be made to work for cross. As
much as I think splicing is an evil trick, I think it's best to do this
and at least have it work consistently for now.
It would seems simpler to expose a `newScopeWithSplicing`, but there's a
few attrs (like `buildPackages` or `buildHaskellPackages`) that
shouldn't be spliced. Users should instead splice, override the splicing
on those packages, and apply `newScope` to that.
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