Only take the first record from ghc-pkg query in haste-copy-pkg. Fixes #179

Bug 179 (in my case, at least) was caused by ghc-pkg describe describing all installed instances of a package. With this patch, we only take the first instance of the package. I don't know off the top of my head that the first one is always the one that cabal-install would default to use, but it is at least better than creating mangled package.conf files.