The Problem
-----------

When I started looking at the problem of providing CI for `head.hackage`
I considered two possible designs:

 1. Build upon `cabal-install`
 2. Build upon Nix's Haskell infrastructure

While I preferred (1), I found that integrating with `cabal-install` was
quite difficult:

 * it [does not produce
   logs](https://github.com/haskell/cabal/issues/5901) for local packages,
   which was the obvious way to incorporate patched packages into the
   build plan

 * it is difficult to reconstruct why a package build failed (e.g. due
   to a planning failure, dependency failing to build, or an error in
   the package itself)

For these reasons it so happened that (2) ended up being a tad easier to
implement. However, it suffers from a number of problems:

 * Nix's Haskell infrastructure doesn't handle multiple versions of a
   single package at all, yet we now have patches for multiple package
   versions in `head.hackage`

 * Nix's Haskell infrastructure doesn't handle flags, which can
   complicate building some packages

 * The Nix expressions ended up being rather difficult to maintain

The Solution
------------

This MR moves the CI infrastructure back in the direction of (1),
facilitated by workarounds that I found for the two issues described
above.

The infrastructure revolves around the `head-hackage-ci` executable
which provides a `test-patches` mode which `gitlab-ci.yml` invokes
thusly:
```
head-hackage-ci test-patches --patches=./patches --with-compiler=$GHC
```
This mode does several things:

 1. Builds a local package repository (using the same script used to
    build `https://ghc.gitlab.haskell.org/head.hackage/`). (N.B. by
    pulling patched packages from a proper repository instead of using
    local packages we side-step the fact that `cabal-install` doesn't
    produce logs for local packages)
 2. Generate a `cabal.project` file containing:

    * a `remote-repository` stanza referring to this repository
    * constraints to ensure that we only choose patched package versions
    * some additional `package` stanzas to ensure that `Cabal` can find
      native library dependencies (these are defined in
      `ci/build-deps.nix`)

 3. Run `cabal new-update` (as well as perform a dummy build of the
    `acme-box` package to ensure that the package index cache is built,
    otherwise parallel builds can randomly fail)
 4. For each patched create a new working directory containing:

    * the previously generated `cabal.project` file
    * a `test-$PKGNAME.cabal` file defining a dummy package depending
      upon the library

    and perform the build. We use some heuristics depending upon:

    * the `plan.json` file
    * which log files exist
    * the contents of said log files

    to sort out what happened.
 6. After all the packages have been built produce a final report of the
    result.

While this is admittedly pretty hacky, in truth it's no worse than the
somersaults which we had to perform in the Nix infrastructure. Reliably
introspecting on failed builds seems to be messy business no matter
which build system you use.

Fixes #7.

- v2-update is preferred (i.e. `v2-` prefix)
- but it's actually broken, due still existing `cabal-install` bug in
  non-append repository updates
- Also add explicit link to haskell.gitlab.com