Merge.hs: use the same constraints for ghc's core package when resolving as when guessing

Fixes bug in misguessing 'bytestring-in-base' flag
noticed by Ke.

https://github.com/gentoo-haskell/gentoo-haskell/commit/a139a31d007607da890adb526d8638f88b1e4241

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>