[xmonad] Change build directory to ~/.cache/xmonad

[Daniel Trstenjak]

> If you look a tad further down the patch list, you'll see it ends in
> ~/.cache/xmonad, presumably following the XDG spec, which was
> designed exactly to avoid namespace pollution.

What's the darcs way for cleaning up a set of patches?

In git I would have done a 'git rebase --interactive' to clean up the patch set.

I know some might cry out because of the history rewriting in git, but
sometimes it's just handy.