Support aeson >=2 about cabal-plan

[phadej]

This seems to require actual code changes

[1 of 1] Compiling Cabal.Plan       ( src/Cabal/Plan.hs, /code/shared-haskell/cabal-plan/dist-newstyle/build/x86_64-linux/ghc-8.6.5/cabal-plan-0.7.2.1/build/Cabal/Plan.o )

src/Cabal/Plan.hs:373:20: error:
    • Couldn't match expected type ‘Key’ with actual type ‘Text’
    • In the second argument of ‘(.:?)’, namely ‘fld’
      In the first argument of ‘(.!=)’, namely ‘o .:? fld’
      In the expression: o .:? fld .!= mempty
    |
373 | o .:?! fld = o .:? fld .!= Data.Monoid.mempty
    |