add System.FilePath to this conversion

It seems to make sense to convert both System.Directory and
System.FilePath uses to OsPath in one go. This will generally look like
replacing RawFilePath with OsPath in type signatures, and will be driven
by the now absolutely massive pile of compile errors.

Got a few modules building in this new regime.

Sponsored-by: Jack Hill

Build/TestConfig.hs

@@ -11,7 +11,6 @@ import Utility.SystemDirectory
 import Utility.OsPath
 
 import System.IO
-import System.FilePath
 
 type ConfigKey = String
 data ConfigValue =
@@ -106,8 +105,11 @@ findCmdPath k command = do
 		)
   where
 	find d =
-		let f = d </> command
-		in ifM (doesFileExist (toOsPath f)) ( return (Just f), return Nothing )
+		let f = toOsPath d </> toOsPath command
+		in ifM (doesFileExist f)
+			( return (Just (fromOsPath f))
+			, return Nothing
+			)
 
 quiet :: String -> String
 quiet s = s ++ " >/dev/null 2>&1"