Unbreak the world target

I'd accidentally left out the world target from the parsing phase
of the new user target handling system, so cabal install world did
not work. Now added to the target parser.

cabal-install/Distribution/Client/Targets.hs

@@ -202,13 +202,17 @@ data UserTargetProblem
    | UserTargetUnexpectedUriScheme String
    | UserTargetUnrecognisedUri     String
    | UserTargetUnrecognised        String
+   | UserTargetBadWorldPkg
   deriving Show
 
 readUserTarget :: String -> IO (Either UserTargetProblem UserTarget)
 readUserTarget targetstr =
     case testNamedTargets targetstr of
-      Just target -> return target
-      Nothing     -> do
+      Just (Dependency (PackageName "world") verrange)
+        | verrange == anyVersion -> return (Right UserTargetWorld)
+        | otherwise              -> return (Left  UserTargetBadWorldPkg)
+      Just dep                   -> return (Right (UserTargetNamed dep))
+      Nothing -> do
         fileTarget <- testFileTargets targetstr
         case fileTarget of
           Just target -> return target
@@ -217,8 +221,7 @@ readUserTarget targetstr =
               Just target -> return target
               Nothing     -> return (Left (UserTargetUnrecognised targetstr))
   where
-    testNamedTargets = fmap (Right . UserTargetNamed)
-                     . readPToMaybe parseDependencyOrPackageId
+    testNamedTargets = readPToMaybe parseDependencyOrPackageId
 
     testFileTargets filename = do
       isDir  <- doesDirectoryExist filename
@@ -292,6 +295,10 @@ reportUserTargetProblems problems = do
              ++ " - cabal files 'pkgname.cabal' or package directories 'pkgname/'\n"
              ++ " - package tarballs 'pkgname.tar.gz' or 'http://example.com/pkgname.tar.gz'"
 
+    case [ () | UserTargetBadWorldPkg <- problems ] of
+      [] -> return ()
+      _  -> die "The special 'world' target does not take any version."
+
     case [ target | UserTargetNonexistantFile target <- problems ] of
       []     -> return ()
       target -> die