Shared: add uriPathToPath.

This adjusts the path from a file: URI in a way that is sensitive
to Windows/Linux differences.  Thus, on Windows,
`/c:/foo` gets interpreted as `c:/foo`, but on Linux,
`/c:/foo` gets interpreted as `/c:/foo`.

See #4613.

src/Text/Pandoc/Shared.hs

@@ -84,6 +84,7 @@ module Text.Pandoc.Shared (
                      -- * File handling
                      inDirectory,
                      collapseFilePath,
+                     uriPathToPath,
                      filteredFilesFromArchive,
                      -- * URI handling
                      schemes,
@@ -635,6 +636,19 @@ collapseFilePath = Posix.joinPath . reverse . foldl go [] . splitDirectories
     isSingleton _   = Nothing
     checkPathSeperator = fmap isPathSeparator . isSingleton
 
+-- Convert the path part of a file: URI to a regular path.
+-- On windows, @/c:/foo@ should be @c:/foo@.
+-- On linux, @/foo@ should be @/foo@.
+uriPathToPath :: String -> FilePath
+uriPathToPath path =
+#ifdef _WINDOWS
+  case path of
+    '/':ps -> ps
+    _      -> p
+#else
+  path
+#endif
+
 --
 -- File selection from the archive
 --