Clean up remaining bits related to ghc-cabal

[snowleopard]

After merging #531, we still have a few occurrences of the ghc-cabal package, as well as the GhcCabal builder, which we need to clean up. See #635.

[snowleopard]

@alpmestan @angerman @chitrak7 I think I found a good way to merge GhcCabal and CabalFlags. I replaced them with a new builder Cabal with ConfigurationInfo = Setup | Flags to distinguish between the two pieces of information -- see the latest commit. Any comments/suggestions?

[angerman]

Look good. I’m still stumbling over the Setup name though; can’t think of a better name right now though. Something along the line of SetupHsArgs might be more descriptive.

[snowleopard]

@angerman Thanks! Yes, I might rename Setup to something more descriptive -- it's not used a lot, so there is not a lot of gain from using a short name.

By the way, could you clarify the purpose of GhcPkg Dependencies builder? It is used in copyConf to extract dependencies from a .conf file, but I don't understand how this is different from other available dependency extraction mechanisms, e.g. we have PackageData.dependencies and PackageData.depIpIds.