Replace expensive unique-like implementations

https://github.com/haskell/cabal/blob/c3c060f4b77633b227378384ff397de09909af1d/Cabal-tests/tests/UnitTests/Distribution/PackageDescription/Check.hs#L29

Spotted by @Bodigrim when reviewing #11825. Please see the thread starting with https://github.com/haskell/cabal/pull/11825#discussion_r3244945542 for more detail.

Functions to check and mentioned in that thread are the above and the following;

https://github.com/haskell/cabal/blob/e7aaad898284c85fe566dd7edc738bb37c1e77c2/Cabal-tests/tests/UnitTests/Distribution/Compat/Graph.hs#L64-L71

https://github.com/haskell/cabal/blob/e7aaad898284c85fe566dd7edc738bb37c1e77c2/cabal-install/tests/UnitTests/Distribution/Client/UserConfig.hs#L93-L96

https://github.com/haskell/cabal/blob/e7aaad898284c85fe566dd7edc738bb37c1e77c2/cabal-install/tests/UnitTests/Distribution/Solver/Modular/WeightedPSQ.hs#L52-L54

https://github.com/haskell/cabal/blob/e7aaad898284c85fe566dd7edc738bb37c1e77c2/solver-benchmarks/HackageBenchmark.hs#L351-L352

https://github.com/haskell/cabal/blob/e7aaad898284c85fe566dd7edc738bb37c1e77c2/cabal-install-solver/src/Distribution/Solver/Modular/Linking.hs#L518-L521