Instantiate creates the package instance (so there is a package_id and package specific parameters, etc) but does
not create a site_node. Mount is the act of associating a
package with a site node. Note that some package instances are not mounted at all and some are mounted multiple times
(the same instance). The most common case is that you
both instantiate and mount at the same time (which is why
there is an instantiate and mount callback).
In this case, if you wanted to do all that work in a callback
I would do it in the after mount callback.
