use haskell-cabal helpers, not ghc-package ones

Commit 16d50ab5453e9d2d43c5292bfb49ddcf152bf870 from
gentoo-haskell overlay stopped relying on implicit
phase function export from ghc-package.eclass.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Portage/EBuild.hs

@@ -148,7 +148,7 @@ showEBuild now ebuild =
             ss "}" . nl).
 
   if_games (nl . ss "pkg_postinst() {" . nl.
-            ss (tabify_line " ghc-package_pkg_postinst") . nl.
+            ss (tabify_line " haskell-cabal_pkg_postinst") . nl.
             ss (tabify_line " games_pkg_postinst") . nl.
             ss "}" . nl).