Package resources with `pkgutil`, `pkg_resources` and `importlib.resources`

I've tested several mechanisms for package resources with in-memory PyOxidizer builds and the results are inconsistent. I made a little demo repository where I try out different mechanisms: https://github.com/thijsmie/pyoxidizer-resource-tests I hope this can serve as a reproducer while debugging this.

A short summary would be that `pkgutil` never works, `importlib.resources` works in some cases and `pkg_resources` always works in my tests so far. This is probably related to #529.

Start with the linked pyoxidizer-resource-tests demo repository and reproduce its in-memory build cases for pkgutil, importlib.resources, and pkg_resources. Trace the corresponding PyOxidizer resource handling to determine why the mechanisms differ. Done means a documented diagnosis and, if the implementation is responsible, consistent behavior across the reported cases.

Written by the indexing model from the issue text.