Add project-plan command

You can force project planning by using `cabal build --dry`. That command is however requires that you in fact have build environment set up. The `project-plan` should be able to take all required information as separate input (compiler information, installed package info db, pkgconfig db, ...).

`project-plan` would both useful for system independent **testing** (simulating various scenarios, where setting up environment is just too tricky for CI), and we can use it to produce plans for cabal's new bootstrap (#6700).

cc @angerman @bgamari

I suspect that this will require some non-trivial changes inside `cabal-install`, but I do think they will be for better, making implicits explicit.