Include standalone haddock derivation for a project

[michaelpj]

This would require a bit of hacking, but I think probably worth it (see e.g. https://github.com/input-output-hk/haskell.nix/issues/735).

We can copy my hack in https://github.com/input-output-hk/plutus/blob/master/nix/haddock-combine.nix, however I think we should:

• Rewrite it in a better language (maybe just Python)
• Deal with the remaining store references (optionally?)

For the latter one, I can see two approaches:

1. Copy any store paths which are referenced in URLs into a "fake store" directory in the bundle, and rewrite any paths to point to that directory. We'd have to do this repeatedly, and it would use up lots of space, but if we want it to be standalone I think it's what we need to do.
2. Point them to Hackage where possible (maybe fall back to 1. if we can't find the package version).

---

Related issues so I remember:

Support in cabal: https://github.com/haskell/cabal/issues/5383 Support in haddock: https://github.com/haskell/haddock/issues/1086, https://github.com/haskell/haddock/issues/1126 Existing standalone-haddock tool is outdated: https://github.com/ktvoelker/standalone-haddock/issues/30

[michaelpj]

Maybe https://github.com/haskell/haddock/pull/1277 will help if/when it gets in.

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[michaelpj]

Could use https://github.com/haskell/cabal/pull/8162 now, potentially. Although I don't think it's in a released cabal version.

[michaelpj]

That probably also needs a newer haddock, so may only work with new enough GHCs.

[michaelpj]

We probably can't actually use haddock-project, because that will try and build the haddocks itself. But we should be able to mimic what it does, starting from about here: https://github.com/haskell/cabal/blob/master/cabal-install/src/Distribution/Client/CmdHaddockProject.hs#L252