GF.Infra.Modules: minor tweaks

Still keeping the modules both in a list and in a finite map. The overhead is
smaller than I initially thought.

src/compiler/GF/Grammar/Grammar.hs

@@ -56,7 +56,7 @@ import qualified Data.ByteString.Char8 as BS
 -- | grammar as presented to the compiler
 type SourceGrammar = MGrammar Info
 
-emptySourceGrammar = mGrammar []
+emptySourceGrammar = emptyMGrammar
 
 type SourceModInfo = ModInfo Info