Fix package name in ghc-pkg.el

The public name of this package is "ghc", based on the package names on both Marmalade and Melpa, and on the name of the ghc-pkg.el file. This commit fixes the contents of the -pkg.el file so that the name specified therein matches the external package name.
Merge pull request #125 from purcell/patch-1

Fix package name in ghc-pkg.el