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

Annex/Import.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
 
 module Annex.Import (
 	ImportTreeConfig(..),
@@ -68,8 +69,10 @@ import Backend.Utilities
 import Control.Concurrent.STM
 import qualified Data.Map.Strict as M
 import qualified Data.Set as S
-import qualified System.FilePath.Posix.ByteString as Posix
 import qualified Data.ByteArray.Encoding as BA
+#ifdef mingw32_HOST_OS
+import qualified System.FilePath.Posix as Posix
+#endif
 
 {- Configures how to build an import tree. -}
 data ImportTreeConfig
@@ -428,8 +431,12 @@ buildImportTreesGeneric converttree basetree msubdir importable@(ImportableConte
 		-- Full directory prefix where the sub tree is located.
 		let fullprefix = asTopFilePath $ case msubdir of
 			Nothing -> subdir
-			Just d -> toOsPath $
-				fromOsPath (getTopFilePath d) Posix.</> fromOsPath subdir
+			Just d ->
+#ifdef mingw32_HOST_OS
+				toOsPath $ fromOsPath (getTopFilePath d) Posix.</> fromOsPath subdir
+#else
+				getTopFilePath d </> subdir
+#endif
 		Tree ts <- converttree (Just fullprefix) $
 			map (\(p, i) -> (mkImportLocation p, i))
 				(importableContentsSubTree c)
@@ -1091,7 +1098,11 @@ getImportableContents r importtreeconfig ci matcher = do
 			isknown <||> (matches <&&> notignored)
 	  where
 		-- Checks, from least to most expensive.
+#ifdef mingw32_HOST_OS
 		ingitdir = ".git" `elem` Posix.splitDirectories (fromOsPath (fromImportLocation loc))
+#else
+		ingitdir = literalOsPath ".git" `elem` splitDirectories (fromImportLocation loc)
+#endif
 		matches = matchesImportLocation matcher loc sz
 		isknown = isKnownImportLocation dbhandle loc
 		notignored = notIgnoredImportLocation importtreeconfig ci loc