The installer runs `get_dependent_ids`, which follows edges outside the
subdag that's being installed, so it returns a superset of the actual
dependents.
That's generally fine, except that it calls `s.package` on every
dependent, which triggers a package class to be instantiated, which is a
lot of work.
Instead, compute the package id from the spec, since that's all that's
used anyways and does not trigger *lots* of slow and redundant
instantiations of package objects.