Infer copyright from LICENSE file

[sol]

We already do this for the license itself, so extending this to copyright is a natural continuation of DRYing up things.

[sol]

An other option would be to generate the LICENSE file from license and copyright. In a way this would be more convenient, as you could maintain all the information in package.yaml to get an up-to-date LICENSE file.

I certainly like it. However, it is less consistent with how hpack currently operates: Currently we look at package.yaml + other source files and generate a .cabal file. That is, currently we don't generate any other files but .cabal. If we break with this convention, we could even consider to generate things like Setup.lhs or even add extension points for third-party plug-ins (think test/Spec.hs for hspec-discover). But not sure yet if this is the right direction.

Feedback welcome!