Take better advantage of cabal nix-style builds

If I figured out a few issues, I could switch to cabal's nix-style builds, and it would have a few advantages. The basic idea is:

1. Specify the environment (e.g., available packages) where client code should run, hopefully in a cabal project file?
2. Let `cabal v2-install` write a GHC environment file on build that describes the exact set of build artifacts that implement that environment. Pass this environment file to GHCJS when building client code.

The more formal specification of the environment (instead of "whatever packages happen to be installed right now") is its own reward. But I also expect this to make builds much faster and more reproducible. Currently, I have a tendency to sometimes hand-pick only some packages to rebuild, to save time in the development cycle, and `cabal v2-install` should make that obsolete.

Challenge: `cabal v2-install` really doesn't seem to like packages just sitting in a subdirectory. It wants them to come from some repository. I'll have to workaround this somehow.