divide packages into "core" and "extra" packages

divide packages into "core" and "extra" packages
The following packages are now "core" packages:

  base, Cabal, haskell98, readline, regex-base, regex-compat
  regex-posix, stm, template-haskell, unix, Win32

Core packages are those packages required to bootstrap GHC, or are
closely tied to GHC (stm, template-haskell).  These are the packages
that will be provided in a source distribution from now on.

All other packages are classified as "extra" packages.  As far as
binary distributions and nightly builds go, nothing will change -
we'll still build and include all these packages in the distributions.

NOTE: 'sh darcs-all get' will now get the core packages only.  To get
the extra packages too, use 'sh darcs-all --extra get'.