fix importing submodules in pythonizations

[m-fila]

BEGINRELEASENOTES

• Fix pythonizations trying to import local modules as pythonizations submodules

ENDRELEASENOTES

load_pythonizations function was by accident sometimes trying to import some local files of downstream projects as submodules of pythonization module (key4hep/EDM4hep#290 (comment))

The culprit is the function pkgutil.walk_modules that tries to recursively find submodules on a given path. utils is a legit submodule of pythonizations found by walk_modules, but then it will try to recursively find submodules somehow including utils also among the local modules if present. I'm not sure whether this is intended or actual bug in pkgutils

Anyway I replace the walk_packages with iter_modules that doesn't have this weird recursive behaviour. The "downside" is that now the pythonizations must be put directly in pythonizations directory with no nesting (which was the case anyway)

[tmadlener]

Merging this. Then key4hep/EDM4hep#290 should pick it up in the CI workflows