cabal2nix: support cabal-install 3.10.1.0 XDG paths.

From 3.10.1.0 onwards, cabal-install uses XDG directories to store its
state and cache components.  cabal2nix expects to find the Hackage
tarball in ~/.cabal, where it may no longer be located.

This commit changes cabal2nix to look in ~/.cabal if that directory
exists (which matches the behaviour of cabal-install itself), but
otherwise looks in the appropriate XDG directory.

hackage-db/src/Distribution/Hackage/DB/Path.hs

@@ -17,8 +17,14 @@ import Control.Exception
 import System.Directory
 import System.FilePath
 
+-- Some logic to handle both old ~/.cabal and XDG cache directory used
+-- by default from cabal-install 3.10.1.0.
 cabalStateDir :: IO FilePath
-cabalStateDir = getAppUserDataDirectory "cabal"
+cabalStateDir = do
+  dotCabal <- getAppUserDataDirectory "cabal"
+  dotCabalExists <- doesDirectoryExist dotCabal
+  if dotCabalExists then pure dotCabal else
+    getXdgDirectory XdgCache "cabal"
 
 cabalTarballDir :: String -> IO FilePath
 cabalTarballDir repo = do