libexec/xbps-src-dopkg.sh: make "meta" build_style pkgs to use do_xxx and pkg_install funcs.
Added support to declare vpkg build deps in templates (close #15).

The current syntax is "<realpkg>?<vpkg>".

Where "<realpkg>" is the real pkgname of the virtual pkg to be built
and "<vpkg>" is the virtual pkgname that should be stored into the binary pkg.
Bump to v69.