Nix: Select the correct GHC version in more cases.

[mboes]

Even when abstract resolver is provided.

Fixes #1641.

[mboes]

Basic approach not workable. Will resubmit once I get some time to think about this a bit more.

[borsboom]

What turned out to be unworkable about the approach? This is relevant for Docker as well.

[mboes]

What didn't work is that I completely forgot to take into the stack.yaml resolver. The needed info is available in the Project datatype passed to nixOptsFromMonoid and dockerOptsFromMonoid, but not in reexecWithOptionalShell.

So what we need is this: create a closure in nixOptsFromMonoid that chooses a compiler package name based on the Project info by default. Store the closure as a field in NixOpts. Then, if in reality the user passed in an abstract resolver on the command-line, then concretize it in reexecWithOptionalShell and invoke the closure to get a final compiler package name. That's exactly what I did in #1646, which is a slight rewrite of this PR.