Build Stack with Cabal-3.0

[hsyl20]

We should build Stack against Cabal 3.* once it is released.

[hsyl20]

For some reason I've been trying to compile Stack against the not-yet released Cabal 3.0.

To avoid effort duplication, you can find my branch here: https://github.com/hsyl20/stack/tree/hsyl20-cabal3

[recursion-ninja]

stack is incompatible with projects that use features added to the .cabal file specification in Cabal-3.0. It would be useful to prioritize this so stack can build projects using modern feature, such as multiple public libraries.

[mpilgrem]

I am going to close this, because Stack is currently built with a dependency on Cabal-3.8.1.0.