Make Setup a separate component

[sheaf]

Quoting from Distribution.Client.ProjectPlanning.elaborateInstallPlan.elaborateSolverToComponents:

At this point in time, only non-Custom setup scripts are supported. Implementing per-component builds with Custom would require us to create a new ElabSetup type, and teach all of the code paths how to handle it.

And from Distribution.Client.ProjectPlanning.setupHsScriptOptions:

TODO: Fix this so custom is a separate component. Custom can ALWAYS be a separate component!!!

So... what's at stake here? The problem is that packages with a custom setup (which currently includes Custom and Hooks build types) go through the "whole package" codepath in cabal-install, in that elaborateSolverToComponents returns a whole package instead of a collection of individual components. This is because cabal-install only has a half-baked notion of Setup as a separate component (e.g. we have compComponentName :: Maybe ComponentName, where Nothing <=> setup).

This means that packages which use those build types are locked out of certain newer features, such as multiple libraries.

Instead, we should make the setup a separate component, which other components in the package depend on. It would probably also be worth tackling #3984 while in this area, as otherwise we would get incorrect recompilation checking if the setup consists of more than one module.

I believe the core of an implementation would be to figure out how to populate the fields of ElaboratedComponent in Distribution.Client.ProjectPlanning.elaborateInstallPlan (see the notImpl function). Then remove the cuz_buildtype legacy fallback reason and try to build some packages with Hooks build-type (see the SetupHooks overlay for a repository of such packages).