Prevent pre-compilation package from triggering its own extensions (#56666)
It is possible for an extension `ExtAB` to be loadable by one of its
triggers, e.g. if `A` loads `B`. However, this loading is not supposed
to happen during pre-compilation of `A`.
Getting this wrong means disagreeing with the scheduled pre-compile jobs
(`A` is not scheduled to depend on or generate a cache file for `ExtAB`
but accidentally attempts both) and leads to confusing errors about
missing cache files.
We used to cover up this bad behavior w/ an erroneous cycle warning
(fixed by #55910), but now we need to be sure this works.