Investigate PKG_DEL scripts

[cbm755]

its pkg unload's fault for not giving us a hook

Not sure what exactly you need. But the PKG_DEL scripts might do something similar. See: https://docs.octave.org/latest/Creating-Packages.html
Essentially, these scripts are executed whenever the path containing them is removed from Octave's load path. That is happening on pkg unload. (But admittedly, it's not a real hook but just a script.)
Maybe you could use (mlocked) functions that keep some values stored in persistent variables if you need to access them from the PKG_DEL script?

Originally posted by @mmuetzel in #1175 (comment)

[alexvong243f]

In the future, we should investigate whether to trigger cleanup in PKG_DEL script. This could simplify our current approach.