Move to `extraHackages`

[MangoIV]

• as we're moving away from extraSources to extraHackages from https://github.com/mlabs-haskell/haskell-nix-extra-hackage we should also do it here, to make it easier to include plutip as a dependency

[szg251]

The work has started here:
mlabs-haskell/bot-plutus-interface#97

[MangoIV]

thank you, awesome!