Split up Stack.Types.Config

[sjakobi]

(Split off from #2334)

This is very large module that takes a long time to compile for minor changes in itself or in its dependencies.

I said in #2334:

And why are types like PackageIndex not in Stack.Types.PackageIndex?

The structure that I added in #1326 should probably be largely disregarded. I didn't really understand the various types at that time.

@chrisdone said in #2334:

This module takes a really long time to compile. And why are types like PackageIndex not in Stack.Types.PackageIndex?

There might be a cyclic dependency problem. Feel free to normalize it. Long compile times are obstacles to feeling motivation to contribute.

I think that mainly Config, ConfigMonoid and HasConfig should stay where they are but most other types should be moved into different / their own modules.