Introduce datatype for the Cabal index cache

Right now this just wraps the list of cache entries, but this might make it
easier in the future to change the structure of the cache.

cabal-install/Distribution/Client/IndexUtils.hs

@@ -368,7 +368,7 @@ updatePackageIndexCacheFile verbosity index = do
                  . maybeDecompress
                =<< BS.readFile (indexFile index)
     entries <- lazySequence pkgsOrPrefs
-    let cache = map toCache $ catMaybes entries
+    let cache = Cache { cacheEntries = map toCache $ catMaybes entries }
     writeFile (cacheFile index) (showIndexCache cache)
   where
     toCache :: PackageOrDep -> IndexCacheEntry
@@ -398,10 +398,10 @@ readPackageIndexCacheFile mkPkg index mode = do
 packageIndexFromCache :: Package pkg
                       => (PackageEntry -> pkg)
                       -> Handle
-                      -> [IndexCacheEntry]
+                      -> Cache
                       -> ReadPackageIndexMode
                       -> IO (PackageIndex pkg, [Dependency])
-packageIndexFromCache mkPkg hnd entrs mode = accum mempty [] entrs
+packageIndexFromCache mkPkg hnd Cache{..} mode = accum mempty [] cacheEntries
   where
     accum srcpkgs prefs [] = do
       -- Have to reverse entries, since in a tar file, later entries mask
@@ -551,8 +551,15 @@ showIndexCacheEntry entry = unwords $ case entry of
                              , display dep
                              ]
 
-readIndexCache :: BSS.ByteString -> [IndexCacheEntry]
-readIndexCache = mapMaybe readIndexCacheEntry . BSS.lines
+-- | Cabal caches various information about the Hackage index
+data Cache = Cache {
+    cacheEntries :: [IndexCacheEntry]
+  }
+
+readIndexCache :: BSS.ByteString -> Cache
+readIndexCache bs = Cache {
+    cacheEntries = mapMaybe readIndexCacheEntry $ BSS.lines bs
+  }
 
-showIndexCache :: [IndexCacheEntry] -> String
-showIndexCache = unlines . map showIndexCacheEntry
+showIndexCache :: Cache -> String
+showIndexCache Cache{..} = unlines $ map showIndexCacheEntry cacheEntries