under some circumstances, we end up with libexecinfo -> BaseSystem being

under some circumstances, we end up with libexecinfo -> BaseSystem being
recorded as a new dependency.  Even though ForwardDependencies normally
takes care of that, with tags, this is not enough.

(this happens only because libexecinfo was a "tight" dependency, thus
resulting in a large UpdateSet, and when some of the objects did require
tags in the new package, and when the order of things meant that BaseSystem
was considered a bit early).

Since there's no handle at this point, a dirty but efficient test vs
BaseSystem will do (which is not a valid normal package name anyhow)

tested to fix the obnoxious warning landry@ saw, which I was able to
reproduce on a box...