Make cabal2nix repository a monorepo

[sternenseemann]

Before merging

• [x] What happens to the haskell4nix readthedocs?
• [x] distribution-nixpkgs
• [x] language-nix
• [x] hackage-db
• [x] Document, subtreeing process and how to recover the original history using josh.
• [x] Update source-repository, homepage, bug report URL in cabal files

After merging

• [ ] Transfer issues
• [ ] Transfer PRs
• [ ] ~~What happens with the original repos?~~ Archive the original