Add snapshotDirectory convention

convention/src/GHC/Debug/Convention.hs

-module GHC.Debug.Convention (socketDirectory) where
+
+module GHC.Debug.Convention (socketDirectory, snapshotDirectory, ghcDebugDirectory) where
 
 import System.FilePath
 import System.Directory
 
 -- | The default socket directory in which to place unix domain sockets in
--- ghc-debug-stub.
-socketDirectory :: IO FilePath
-socketDirectory = do
+-- ghc-debug-stub. This is currently your XDG data directory.
+ghcDebugDirectory :: IO FilePath
+ghcDebugDirectory = do
     xdgDir <- getXdgDirectory XdgData ""
     return (xdgDir </> "ghc-debug/debuggee/")
+
+socketDirectory :: IO FilePath
+socketDirectory = (</> "sockets")  <$> ghcDebugDirectory
+
+snapshotDirectory :: IO FilePath
+snapshotDirectory = (</> "snapshots")  <$> ghcDebugDirectory