solver: parse cabal errors also on Windows

Noticed on #2502. Basically, `isSuffixOf "suffix" "stuff suffix\r"` is
spuriously false, so we should better strip `\r`.

I've not yet verified that `\r` is actually there; I infer it from the
presence of `stripCR` and the parse failure. Time to test this.

src/Stack/Solver.hs

@@ -136,6 +136,7 @@ cabalSolver menv cabalfps constraintType
 
   where
     errCheck = T.isInfixOf "Could not resolve dependencies"
+    linesNoCR = map stripCR . T.lines
     cabalBuildErrMsg e =
                ">>>> Cabal errors begin\n"
             <> e
@@ -167,7 +168,7 @@ cabalSolver menv cabalfps constraintType
         else errExit msg
 
     parseConflictingPkgs msg =
-        let ls = dropWhile (not . errCheck) $ T.lines msg
+        let ls = dropWhile (not . errCheck) $ linesNoCR msg
             select s = ((T.isPrefixOf "trying:" s)
                       || (T.isPrefixOf "next goal:" s))
                       && (T.isSuffixOf "(user goal)" s)
@@ -180,8 +181,7 @@ cabalSolver menv cabalfps constraintType
     parseCabalOutput bs = do
         let ls = drop 1
                $ dropWhile (not . T.isPrefixOf "In order, ")
-               $ map stripCR
-               $ T.lines
+               $ linesNoCR
                $ decodeUtf8 bs
             (errs, pairs) = partitionEithers $ map parseCabalOutputLine ls
         if null errs