Update Cabal-syntax/src/Distribution/Parsec/Source.hs

Co-authored-by: Phil de Joux <philderbeast@gmail.com>

Author: mpickering

Cabal-syntax/src/Distribution/Parsec/Source.hs

@@ -19,7 +19,7 @@ data PSource src
   | PUnknownSource
   deriving (Ord, Show, Generic, Functor)
 
-data CabalFileSource
+newtype CabalFileSource
   = PCabalFile (FilePath, BS.ByteString)
   deriving (Ord, Show, Generic)