add data-dir field to package config

Cabal will look for data files to install relative to the directory given
in the data-dir field, allowing package authors to better structure their
source tree. There's no behavioural change by default.

Distribution/PackageDescription.hs

@@ -129,6 +129,7 @@ data PackageDescription
         library        :: Maybe Library,
         executables    :: [Executable],
         dataFiles      :: [FilePath],
+        dataDir        :: FilePath,
         extraSrcFiles  :: [FilePath],
         extraTmpFiles  :: [FilePath]
     }
@@ -159,6 +160,7 @@ emptyPackageDescription
                       library      = Nothing,
                       executables  = [],
                       dataFiles    = [],
+                      dataDir      = "",
                       extraSrcFiles = [],
                       extraTmpFiles = []
                      }

Distribution/PackageDescription/Parse.hs

@@ -143,6 +143,9 @@ pkgDescrFieldDescrs =
  , listField "data-files"  
            showFilePath           parseFilePathQ
            dataFiles              (\val pkg -> pkg{dataFiles=val})
+ , simpleField "data-dir"
+           showFilePath           parseFilePathQ
+           dataDir                (\val pkg -> pkg{dataDir=val})
  , listField "extra-source-files" 
            showFilePath    parseFilePathQ
            extraSrcFiles          (\val pkg -> pkg{extraSrcFiles=val})

Distribution/Simple/Install.hs

@@ -104,7 +104,7 @@ install pkg_descr lbi flags = do
   flip mapM_ (dataFiles pkg_descr) $ \ file -> do
       let dir = takeDirectory file
       createDirectoryIfMissingVerbose verbosity True (dataPref </> dir)
-      copyFileVerbose verbosity file (dataPref </> file)
+      copyFileVerbose verbosity (dataDir pkg_descr </> file) (dataPref </> file)
   when docExists $ do
       createDirectoryIfMissingVerbose verbosity True htmlPref
       copyDirectoryRecursiveVerbose verbosity

Distribution/Simple/SrcDist.hs

@@ -150,8 +150,9 @@ prepareTree verbosity pkg_descr mb_lbi tmpDir pps = do
         Nothing -> findFile (hsSourceDirs exeBi) mainPath
         Just pp -> return pp
     copyFileTo verbosity targetDir srcMainFile
-  flip mapM_ (dataFiles pkg_descr) $ \ file -> do
-    let dir = takeDirectory file
+  flip mapM_ (dataFiles pkg_descr) $ \ filename -> do
+    let file = dataDir pkg_descr </> filename
+        dir = takeDirectory file
     createDirectoryIfMissingVerbose verbosity True (targetDir </> dir)
     copyFileVerbose verbosity file (targetDir </> file)