[#90] fix Indigo's tutorial package to make it buildable

Description

Indigo's tutorial does not actually build, because its example modules are not included in the cabal file. Additionally, recent updates have broken these modules so they would not compile anyway.

Related issue(s)

Part of #90 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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• My code complies with the style guide.