packages: Optimize ‘all-packages’....On my laptop, wall-clock time for (all-packages) goes from 27s to 1s.
* gnu/packages.scm (all-packages): Use a hash table to remember visited
packages instead of calling ‘delete-duplicates’ on the final list.
Change-Id: I4aae804656b56ef2095993e91f0572a5891f419f