subr/pkg_setup_distclean.subr: rm(1) ${PKG_BASE_DIR} on-demand.
etc/build.usage: updated.
subr/pkg_setup_{env,subdirs}.subr: split {cd,mkdir}(1) sequence into setup_env_subdirs.
vars/env.vars: adds setup_distclean:variant & setup_subdirs:invariant build steps.