Fix default hyperlinked sources pattern

Previously this didn't include the `%M` token which manifested as broken
links to the hyperlinked sources of reexports of declarations defined
in other packages.

Fixes haddock#1628.

(cherry picked from commit 1432bcc943d41736eca491ecec4eb9a6304dab36)

utils/haddock/haddock-api/src/Haddock/Options.hs

@@ -563,7 +563,7 @@ readIfaceArgs flags = [parseIfaceOption s | Flag_ReadInterface s <- flags]
             (src, ',' : rest') ->
               let src' = case src of
                     "" -> Nothing
-                    _ -> Just src
+                    _  -> Just (src ++ "/%M.html")
                in case break (== ',') rest' of
                     (visibility, ',' : file)
                       | visibility == "hidden" ->