"cabal new-exec ghc" should work outside of .cabal projects

Nearly half my time in Haskell development is spend running `ghc` on individual files, outside of `.cabal` projects, which might have some number of dependencies. Prevously, I could just `old-install` all of my dependencies, point `ghc` to the right file, and everything would Just Work™. Unfortunately, there isn't really a `new-*` equivalent of this. There's `cabal new-exec ghc`, but that assumes that you're in a `.cabal` project, which usually _isn't_ the case for me.

Similarly to how you can run `cabal new-repl -b dep1,...,depn` outside of a `.cabal` project today, I'd like to request that `cabal new-exec -b dep1,...,depn ghc -- Foo.hs` work outside of `.cabal` projects. This would operate conceptually in a similar manner to `stack exec ghc`.

cc @phadej