Factor loadExactConfig out of loadRawConfig

loadExactConfig will load just the config in the given file path or return
nothing if the file does not exist. loadRawConfig will continue to load the user
config in ~/.cabal/config or the file specified by CABAL_CONFIG. loadExactConfig
is factored out so that the same logic may be used to read config files from
other locations.

cabal-install/Distribution/Client/Config.hs

@@ -16,6 +16,7 @@
 module Distribution.Client.Config (
     SavedConfig(..),
     loadConfig,
+    loadExactConfig,
     getConfigFilePath,
 
     showConfig,
@@ -613,31 +614,41 @@ extendToEffectiveConfig config = do
 loadRawConfig :: Verbosity -> Flag FilePath -> IO SavedConfig
 loadRawConfig verbosity configFileFlag = do
   (source, configFile) <- getConfigFilePathAndSource configFileFlag
+  notice verbosity $ "Global config file path source is " ++ sourceMsg source ++ "."
+  minp <- loadExactConfig verbosity configFile
+  case minp of
+    Nothing -> createDefaultConfigFile verbosity configFile
+    Just conf -> return conf
+
+  where
+    sourceMsg CommandlineOption =   "commandline option"
+    sourceMsg EnvironmentVariable = "env var CABAL_CONFIG"
+    sourceMsg Default =             "default config file"
+
+data ConfigFileSource = CommandlineOption
+                      | EnvironmentVariable
+                      | Default
+
+-- | Like 'loadRawConfig', but loads only exactly the file given.
+--
+loadExactConfig :: Verbosity -> FilePath -> IO (Maybe SavedConfig)
+loadExactConfig verbosity configFile = do
   minp <- readConfigFile mempty configFile
   case minp of
     Nothing -> do
-      notice verbosity $ "Config file path source is " ++ sourceMsg source ++ "."
       notice verbosity $ "Config file " ++ configFile ++ " not found."
-      createDefaultConfigFile verbosity configFile
+      return Nothing
     Just (ParseOk ws conf) -> do
+      notice verbosity $ "Config file " ++ configFile ++ " loaded."
       unless (null ws) $ warn verbosity $
         unlines (map (showPWarning configFile) ws)
-      return conf
+      return (Just conf)
     Just (ParseFailed err) -> do
       let (line, msg) = locatedErrorMsg err
       die' verbosity $
           "Error parsing config file " ++ configFile
         ++ maybe "" (\n -> ':' : show n) line ++ ":\n" ++ msg
 
-  where
-    sourceMsg CommandlineOption =   "commandline option"
-    sourceMsg EnvironmentVariable = "env var CABAL_CONFIG"
-    sourceMsg Default =             "default config file"
-
-data ConfigFileSource = CommandlineOption
-                      | EnvironmentVariable
-                      | Default
-
 -- | Returns the config file path, without checking that the file exists.
 -- The order of precedence is: input flag, CABAL_CONFIG, default location.
 getConfigFilePath :: Flag FilePath -> IO FilePath