ci: Simplify usage directions

While discussing usage of head.hackage with Simon it was pointed out that it would be simpler for users if there were a single command which could be invoked to add head.hackage to a project. Here we refactor the cabal.project snippets to make this feasible. In particular, we now provide three files:

• repo.project contains the minimal snippet which just defines the source-repository stanza
• cabal.constraints contains the version constraints and allow-newer declaration
• cabal.project is the concatenation of both

We can then instruct users to simply curl cabal.project into their cabal.project.local file.