Allow packages to call default prep/compile

 * renamed pkg_prep() and pkg_compile() to build_prep() and build_compile().
   The new pkg_*() routines simply call these builtins.  This allows packages
   to utilize the defaults w/out reinventing them.
Improve export filtering

 * We remove more "dev" items from the exported rpm by default now.  In the
   long run we should be dividing this data into dev rpm's and dbg rpm's.