Build nonReinstallablePkgs from pre-existing packages in plan.json

[hamishmack]

Haskell.nix now respects the pre-existing packages selected by the cabal planner. The selection made by the planner is used to set nonReinstallablePkgs.

Instead setting nonReinstallablePkgs and reinstallableLibGhc haskell.nix projects should add constraints to the cabal project.

For instance to force the use of the pre-exising text package add:

  constraints: text installed

To make sure text is reinstalled use:

  constraints: text source

The pre-existing ghc will now be used by default as that is what cabal will choose (haskell.nix used to choose reinstallableLibGhc=true by default).

To allow cabal to choose reinstalling ghc add:

  allow-boot-library-installs: True

To force cabal to choose reinstalling:

  constraints: ghc source
  allow-boot-library-installs: True

It may also need allow-newer: ghc:Cabal