Refactoring in GF.Compile and GF.ReadFiles with an eye to parallel compilation

In particular, the function compileOne has been moved to the new module
GF.CompileOne and its type has been changed from

    compileOne :: ... -> CompileEnv -> FilePath -> IOE CompileEnv

to

    compileOne :: ... -> SourceGrammar -> FilePath -> IOE OneCompiledModule

making it more suitable for use in a parallel compiler.

src/compiler/GF/Infra/UseIO.hs

@@ -105,9 +105,26 @@ getSubdirs dir = do
                        then return (fpath:fs)
                        else return        fs ) [] fs
 
+--------------------------------------------------------------------------------
 justModuleName :: FilePath -> String
 justModuleName = dropExtension . takeFileName
 
+isGFO :: FilePath -> Bool
+isGFO = (== ".gfo") . takeExtensions
+
+gfoFile :: FilePath -> FilePath
+gfoFile f = addExtension f "gfo"
+
+gfFile :: FilePath -> FilePath
+gfFile f = addExtension f "gf"
+
+gf2gfo :: Options -> FilePath -> FilePath
+gf2gfo = gf2gfo' . flag optGFODir
+
+gf2gfo' gfoDir file = maybe (gfoFile (dropExtension file))
+                            (\dir -> dir </> gfoFile (takeBaseName file))
+                            gfoDir
+--------------------------------------------------------------------------------
 splitInModuleSearchPath :: String -> [FilePath]
 splitInModuleSearchPath s = case break isPathSep s of
   (f,_:cs) -> f : splitInModuleSearchPath cs