Introducing cabal-helper-wrapper

The idea here is to build cabal-helper with whatever version of Cabal
the user happens to be using (which we find by looking at
dist/setup-config) at runtime.

This way we can support literally any version of Cabal as long as the
actual cabal-helper still compiles.

I tried to only use interfaces in Cabal that have been there since at
least 1.16 so I'm hoping this shouldn't break too much.

src/GHCModCabal.hs

@@ -1,28 +0,0 @@
-{-# LANGUAGE BangPatterns #-}
-module Main where
-
-import Control.Applicative
-
-import Distribution.Simple.Utils (cabalVersion)
-import Distribution.Simple.Configure
-import Distribution.Text ( display )
-import System.Environment
-import System.Directory
-
-main :: IO ()
-main = do
-  args <- getArgs
-  case args of
-    "version":[] -> do
-        putStrLn $ "using version " ++ display cabalVersion ++ " of the Cabal library"
-    "print-setup-config":args' -> do
-        mfile <- findFile ["dist"] "setup-config"
-
-        let file = case mfile of
-                     Just f -> f
-                     Nothing -> let !(f:[]) = args' in f
-
-        putStrLn =<< show <$> getConfigStateFile file
-
-    cmd:_ -> error $ "Unknown command: " ++ cmd
-    [] -> error "No command given"