community/cabal-stage0: build without internet access
Nowadays, Cabal's bootstrap.py script supports bootstrapping from a source tarball. This allows us to build cabal-stage0 without internet access. Furthermore, the JSON files used for bootstrapping are always hopelessly outdated. Thereby, often requiring us to commit custom bootstrap files to aports (67d7bb6e for example). By bootstrapping from a source tarball instead, we can keep that external.
Creation of the bootstrap source tarballs is described here: https://github.com/haskell/cabal/tree/master/bootstrap#readme