Adjust the URL of index to work with Hackage 2.

Signed-off-by: Magnus Therning <magnus@therning.org>
Make the index URL and filename into variables.

The recent switch to Hackage 2 highlighted that it was time to collect the
index URL and filename into two variables in Util.Misc.

Signed-off-by: Magnus Therning <magnus@therning.org>