constraints: actually forbid the unwanted packages

Intent of "forbidden packages" constraint in `cabal.project` is not
effected.  The current constraint does not prevent inclusion in
build plan - it only ensures the packages are build without the
`none` flag.

Change to a `< 0` version constraint to actually prevent the
unwanted packages from being used.

Author: frasertweedale

cabal.project

@@ -9,8 +9,8 @@ packages:
 -----------------------------------------------------------------------------
 -- Anti-constraints
 
--- make sure we don't accidentally pick up blacklisted packages
-constraints: foundation -none, cryptonite -none, cryptohash -none
+-- make sure we don't accidentally pick up forbidden packages
+constraints: foundation < 0, cryptonite < 0, cryptohash < 0
 
 -----------------------------------------------------------------------------
 -- Modded lib:Cabal