make butido aware of metapackages

[christophprokop]

A metapackage is a list of software (e.g. for one customer) which are not necessarrily dependent on each other but belong together.
At the moment we just declare these lists as runtime dependencies of a metapackage which is fine.
But in the last step butido installs all dependencies in a new container which might take some time (if the list is long) but usually does not provide any benefit. It may sometimes be desired to have this step but much more often its not.

I guess a config option in the pkg.toml file like
is_meta_pkg = 1
would suffice here.

If you want the final container with all the deps installed something like:
butido build --install-meta-deps
could be a ok. We can of course discuss this.

Making butido aware of meta-pkgs would/could also solve #336