ghc-pkg: Allow unregistering multiple packages in one call
This can save lots of time when the cost to invoke a single GHC command is high, for example through docker.
An example user of this would be
stack build, which will unregister all relevant packages if e.g.
--extra-lib-dirs has changed. Currently it has to do that through multiple
ghc-pkg unregister operations, which can take about 2 seconds per call when done through a docker image. If we could unregister multiple packages at once, such use cases would have to pay the 2 seconds penalty only once.