virtual resources should be tagged as virtual

The doc at http://docs.puppetlabs.com/puppet/2.7/reference/lang_relationships.html#chaining-arrows suggests we can write:
Yumrepo <| |> -> Package <| |>
This is typically something one would have as a global default in his `site.pp`.

As a side effect, it will `realize()` every `@yumrepo` and `@package` virtual resource, as mentioned in http://docs.puppetlabs.com/puppet/2.7/reference/lang_virtual.html

This makes chaining arrows and virtual resources a bit of a nuisance to use together.

It would be great if we could write something like instead:
Yumrepo <| virtual == false |> -> Package <| virtual == false |>
or:
Yumrepo <| tag != virtual |> -> Package <| tag != virtual |>

The current workaround is to explicitly tag every virtual resource, which is not very nice.

Another solution would be to not realize virtual resources when `<| |>` is part of a chained arrow, but I guess it's done that way for a purpose ?