sanitise naming of package lists

The *predicates* all start with "PKGS_THAT_...", e.g.:

  PKGS_THAT_BUILD_WITH_STAGE0   (previously "PACKAGES_STAGE0")
  PKGS_THAT_BUILD_WITH_STAGE2   (previously "STAGE2_PACKAGES")
  PKGS_THAT_USE_TH              (previously "TH_PACKAGES)
  etc. (there are a few more)

the lists of packages to build are now consistently named:

  PACKAGES_STAGE0
  PACKAGES_STAGE1  (previously just "PACKAGES")
  PACKAGES_STAGE2