OsPath build flag no longer depends on filepath-bytestring

However, filepath-bytestring is still in Setup-Depends.
That's because Utility.OsPath uses it when not built with OsPath.
It would be maybe possible to make Utility.OsPath fall back to using
filepath, and eliminate that dependency too, but it would mean either
wrapping all of System.FilePath's functions, or using `type OsPath = FilePath`

Annex.Import uses ifdefs to avoid converting back to FilePath when not
on windows. On windows it's a bit slower due to that conversion.
Utility.Path.Windows.convertToWindowsNativeNamespace got a bit
slower too, but not really worth optimising I think.

Note that importing Utility.FileSystemEncoding at the same time as
System.Posix.ByteString will result in conflicting definitions for
RawFilePath. filepath-bytestring avoids that by importing RawFilePath
from System.Posix.ByteString, but that's not possible in
Utility.FileSystemEncoding, since Setup-Depends does not include unix.
This turned out not to affect any code in git-annex though.

Sponsored-by: Leon Schuermann

Utility/Path/Windows.hs

@@ -15,9 +15,10 @@ module Utility.Path.Windows (
 import Utility.Path
 import Utility.OsPath
 import Utility.SystemDirectory
+import Utility.FileSystemEncoding
 
 import qualified Data.ByteString as B
-import qualified System.FilePath.Windows.ByteString as P
+import qualified System.FilePath.Windows as WinPath
 
 {- Convert a filepath to use Windows's native namespace.
  - This avoids filesystem length limits.
@@ -36,9 +37,9 @@ convertToWindowsNativeNamespace f
 		-- Make absolute because any '.' and '..' in the path
 		-- will not be resolved once it's converted.
 		cwd <- getCurrentDirectory
-		let p = fromOsPath (simplifyPath (combine cwd (toOsPath f)))
+		let p = simplifyPath (combine cwd (toOsPath f))
 		-- Normalize slashes.
-		let p' = P.normalise p
+		let p' = encodeBS $ WinPath.normalise $ fromOsPath p
 		return (win32_file_namespace <> p')
   where
 	win32_dev_namespace = "\\\\.\\"