Support conditionals in project files

[phadej]

It would be handy, so haskell-ci would work with

packages: foo
if impl(ghc >=9.0)
  packages: foo-new-stuff

project setups, where new stuff requires newer GHC. As haskell-ci looks for tested-with any way, the implementation won't be hard, we can (for packages) ignore the condiotional tree, flattening it.

However as cabal-install-parsers is public interface, I need to know about haskell/cabal#10706 first