Tweak .cabal to avoid unneeded recompilation

Previously when making a change and running `stack test`, it would
compile each module 3 times: for the library, the executable, and the
test suite.

This change more cleanly segregates the hs-source-dirs for each target,
which avoids recompilation. The only source change is moving Main.hs
into its own directory (but it's otherwise unchanged). See also:
<http://stackoverflow.com/questions/6711151/how-to-avoid-recompiling-in-this-cabal-file>

src/PostgREST/Main.hs

@@ -1,87 +0,0 @@
-{-# LANGUAGE CPP #-}
-
-module Main where
-
-
-import           PostgREST.App
-import           PostgREST.Config                     (AppConfig (..),
-                                                       minimumPgVersion,
-                                                       prettyVersion,
-                                                       readOptions)
-import           PostgREST.DbStructure
-
-import           Control.Monad
-import           Data.Monoid                          ((<>))
-import           Data.String.Conversions              (cs)
-import qualified Hasql.Query                          as H
-import qualified Hasql.Session                        as H
-import qualified Hasql.Decoders                       as HD
-import qualified Hasql.Encoders                       as HE
-import qualified Hasql.Pool                           as P
-import           Network.Wai.Handler.Warp
-import           System.IO                            (BufferMode (..),
-                                                       hSetBuffering, stderr,
-                                                       stdin, stdout)
-import           Web.JWT                              (secret)
-import           Data.IORef
-#ifndef mingw32_HOST_OS
-import           Control.Monad.IO.Class               (liftIO)
-import           System.Posix.Signals
-import           Control.Concurrent                   (myThreadId)
-import           Control.Exception.Base               (throwTo, AsyncException(..))
-#endif
-
-isServerVersionSupported :: H.Session Bool
-isServerVersionSupported = do
-  ver <- H.query () pgVersion
-  return $ read (cs ver) >= minimumPgVersion
- where
-  pgVersion =
-    H.statement "SHOW server_version_num"
-      HE.unit (HD.singleRow $ HD.value HD.text) True
-
-main :: IO ()
-main = do
-  hSetBuffering stdout LineBuffering
-  hSetBuffering stdin  LineBuffering
-  hSetBuffering stderr NoBuffering
-
-  conf <- readOptions
-  let port = configPort conf
-      pgSettings = cs (configDatabase conf)
-      appSettings = setPort port
-                  . setServerName (cs $ "postgrest/" <> prettyVersion)
-                  $ defaultSettings
-
-  unless (secret "secret" /= configJwtSecret conf) $
-    putStrLn "WARNING, running in insecure mode, JWT secret is the default value"
-  Prelude.putStrLn $ "Listening on port " ++
-    (show $ configPort conf :: String)
-
-  pool <- P.acquire (configPool conf, 10, pgSettings)
-
-  result <- P.use pool $ do
-    supported <- isServerVersionSupported
-    unless supported $ error (
-      "Cannot run in this PostgreSQL version, PostgREST needs at least "
-      <> show minimumPgVersion)
-    getDbStructure (cs $ configSchema conf)
-
-  refDbStructure <- newIORef $ either (error.show) id result
-
-#ifndef mingw32_HOST_OS
-  tid <- myThreadId
-  forM_ [sigINT, sigTERM] $ \sig ->
-    void $ installHandler sig (Catch $ do
-        P.release pool
-        throwTo tid UserInterrupt
-      ) Nothing
-
-  void $ installHandler sigHUP (
-      Catch . void . P.use pool $ do
-        s <- getDbStructure (cs $ configSchema conf)
-        liftIO $ atomicWriteIORef refDbStructure s
-   ) Nothing
-#endif
-
-  runSettings appSettings $ postgrest conf refDbStructure pool