Made parsePackageIndex and updatePackageIndexCacheFile streaming.

The parsePackageIndex wasn't streaming because resulting list was
hidden in two constructors: Either and tuple. By removing that error
semantics changed in updatePackageIndexCacheFile.

The updatePackageIndexCacheFile problem was sequence call that evaluated
whole list made by parsePackageIndex. By using lazySequence any error
during index file parsing will be raised during writing to cache file.

Also removed reacPackageIndexFile, as it wasn't used anywhere as far as
I can tell. If this is a mistake I'll include it again.

cabal-install/Distribution/Client/IndexUtils.hs

@@ -288,33 +288,6 @@ packageDesc :: PackageEntry -> GenericPackageDescription
 packageDesc (NormalPackage  _ descr _ _) = descr
 packageDesc (BuildTreeRef _ _ descr _ _) = descr
 
--- | Read a compressed \"00-index.tar.gz\" file into a 'PackageIndex'.
---
--- This is supposed to be an \"all in one\" way to easily get at the info in
--- the Hackage package index.
---
--- It takes a function to map a 'GenericPackageDescription' into any more
--- specific instance of 'Package' that you might want to use. In the simple
--- case you can just use @\_ p -> p@ here.
---
-
-{-
-readPackageIndexFile :: Package pkg
-                     => (PackageEntry -> pkg)
-                     -> FilePath
-                     -> IO (PackageIndex pkg, [Dependency])
-readPackageIndexFile mkPkg indexFile = do
-  (mkPkgs, prefs) <- either fail return
-                     . parsePackageIndex
-                     . maybeDecompress
-                     =<< BS.readFile indexFile
-
-  pkgEntries  <- sequence mkPkgs
-  pkgs <- evaluate $ PackageIndex.fromList (map mkPkg pkgEntries)
-  return (pkgs, prefs)
-
--}
-
 -- | Parse an uncompressed \"00-index.tar\" repository index file represented
 -- as a 'ByteString'.
 --
@@ -413,15 +386,13 @@ updatePackageIndexCacheFile verbosity indexFile cacheFile = do
                  . maybeDecompress
                =<< BS.readFile indexFile
     entries <- lazySequence pkgsOrPrefs
-    let cache = mkCache entries
+    let cache = map toCache entries
     writeFile cacheFile (showIndexCache cache)
   where
     toCache (Pkg (NormalPackage pkgid _ _ blockNo)) = CachePackageId pkgid blockNo
     toCache (Pkg (BuildTreeRef refType _ _ _ blockNo)) = CacheBuildTreeRef refType blockNo
     toCache (Dep d) = CachePreference d
 
-    mkCache = map toCache
-
 data ReadPackageIndexMode = ReadPackageIndexStrict
                           | ReadPackageIndexLazyIO