Implements ./pkgtool.sh -[irt] options.
    
    etc/{README.md,pkgtool.usage}: updated.
    subr/ex_pkg.subr:ex_pkg_{find_package,get_packages}(): initial implementation.
    subr/ex_pkg.subr:ex_pkg_unfold_{,r}depends(): split from ex_pkg_expand_packages().
    subr/ex_pkg{,_dispatch}.subr: removes ${EX_PKG_COMPLETE} scoped global.
    subr/ex_pkg_dispatch.subr:exp_pkg_dispatch_expand_packages(): split from subr/ex_pkg.subr.
    subr/rtl_list.subr:rtl_lsort(): initial implementation.