Extra field for all categories: package + package version

[beerendlauwers]

Currently, there's a general metadata field for "libraries". That should be replaced by a YAML-based metadata field called "packages" that holds a list of tuples of type (PackageName, PackageVersion).

With this, we can identify older and potentially outdated content without having to resort to submission dates, checking whether any code compiles, etc.

EDIT: The supported GHC version could be inferred from the map snd of the list.