PGF library: expose only PGF and PGF.Internal instead of all modules

PGF exports the public, stable API.
PGF.Internal exports additional things needed in the GF compiler & shell,
including the nonstardard version of Data.Binary.

src/compiler/GF/Infra/Option.hs

@@ -43,7 +43,7 @@ import GF.Data.Operations(Err,ErrorMonad(..),liftErr)
 import Data.Set (Set)
 import qualified Data.Set as Set
 
-import PGF.Data(Literal(..))
+import PGF.Internal(Literal(..))
 
 usageHeader :: String
 usageHeader = unlines