This can occur in real cases when etype is related to his parent by multiple
optional relations where only one could be filled. We could generate 'OR'
expressions but for now this is not handled.

* removed backward compat on structure_def location, broken by moving to
  the new layout

* __pkginfo__ and setup are now flake8 compliant

we don't want it to appear auto-magically for every IClonable entities

For instance, following non-composite relations.

Closes #15770380.

Test coverage is currently rather low. A more complex test schema needs to be
created.