Fix `new-sdist` file handle leak

Lazy I/O strikes again.

cabal-install/Distribution/Client/CmdSdist.hs

@@ -72,6 +72,7 @@ import Control.Monad.Writer.Lazy
     ( WriterT, tell, execWriterT )
 import Data.Bits
     ( shiftL )
+import qualified Data.ByteString.Char8      as BS
 import qualified Data.ByteString.Lazy.Char8 as BSL
 import Data.Either
     ( partitionEithers )
@@ -259,7 +260,7 @@ packageToSdist verbosity projectRootDir format outputFile pkg = do
                                 Left err -> liftIO $ die' verbosity ("Error packing sdist: " ++ err)
                                 Right path -> tell [Tar.directoryEntry path]
                             
-                        contents <- liftIO $ BSL.readFile file
+                        contents <- liftIO . fmap BSL.fromStrict . BS.readFile $ file
                         case Tar.toTarPath False (prefix </> file) of
                             Left err -> liftIO $ die' verbosity ("Error packing sdist: " ++ err)
                             Right path -> tell [(Tar.fileEntry path contents) { Tar.entryPermissions = perm' }]

cabal-testsuite/PackageTests/NewSdist/ManyDataFiles/many-data-files.out

+# cabal new-sdist
+Wrote tarball sdist to <ROOT>/many-data-files.dist/source/dist-newstyle/sdist/many-data-files-0.tar.gz

cabal-testsuite/PackageTests/NewSdist/ManyDataFiles/many-data-files.test.hs

@@ -11,5 +11,5 @@ main = cabalTest . withSourceCopy $ do
             liftIO $ createDirectoryIfMissing False (cwd </> "data")
             forM_ [1 .. n + 100] $ \i -> 
                 liftIO $ BS.writeFile (cwd </> "data" </> ("data-file-" ++ show i) <.> "txt") (BS.pack "a data file\n")
-            expectBroken 5541 $ cabal "new-sdist" ["many-data-files"]
+            cabal "new-sdist" ["many-data-files"]
         Nothing -> skip