[#721] HLint rules enforcing module qualification

Note: our nixpkgs has old-ish hlint version still, so I have to build it from hackage.nix. Note hlint 3.4 is actually in NixOS's nixpkgs HEAD, but pulled from there, it doesn't build.

Description

Problem: We have some relatively strict rules about what modules can be imported unqualified without an explicit import list. Those aren't in any way enforced, however.

Solution: Add HLint rules enforcing our import rules to some extent.

Related issue(s)

Resolves #721 (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.

N/A

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.