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/Compile/PGFtoProlog.hs

@@ -9,7 +9,7 @@
 module GF.Compile.PGFtoProlog (grammar2prolog) where
 
 import PGF(mkCId,wildCId,showCId)
-import PGF.Data
+import PGF.Internal
 --import PGF.Macros
 
 import GF.Data.Operations