Hlint: suggest nub alternative

[philderbeast]

Fixes #11824.

When fired, this suggestion renders as:

$ cabal-install-solver/src/Distribution/Solver/Modular/Assignment.hs:56:64-69: Suggestion: Use ordNub
Found:
  nub xs
Perhaps:
  ordNub xs
Note: The growth rate of nub is O(n^2) and for ordNub it is O(n log n).  May require adding import of either;
  - Distribution.Utils.Generic from Cabal-syntax, or
  - Distribution.Simple.Utils from Cabal.

I didn't correct O(n log n) to the O(n log d) from containers haddocks for nubOrd. Neither did I suggest using that equivalent function from containers. For that, I raised #11826.

I had to add Ord instances to a few data types, BuildTarget and MatchError. I changed the Eq constraint to an Ord constraint for findMatch and nubMatches.

I left a note in .hlint.yaml to only use single letter variables in rules.

I used a separate commit for changes to tests but on approval will squash all commits before adding the merge me label.

---

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[jappeace]

nice!