HPath.IO.Errors: fix throwDestinationInSource

'canonicalizePath' was missing, making this function far less reliable.
In order for this to work we have to work around circular imports
with a IO.hs-boot file.

src/HPath/IO.hs-boot

@@ -0,0 +1,7 @@
+module HPath.IO where
+
+
+import HPath
+
+canonicalizePath :: Path Abs -> IO (Path Abs)
+