An easy way to add external files to Haskell source

[srid]

Sometimes, cabal's data-files must come from outside the Git repo, such as from a flake input. To achieve this, normally we:

1. ln -sf that file or directory in devShell's shellHook, and:
2. add it to projectRoot so cabal2nix is aware of it.

In action: https://github.com/juspay/vira/pull/17

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Can we make this simpler to configure in haskell-flake? If not, at least document this as a workflow.