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

* __pkginfo__ and setup are now flake8 compliant

those are copied using standard Entity.copy_relations method, hence adapter's
skiprtypes should be copied in entity's cw_skip_copy_for to control above
method.

that's at least desired by current usage of CloneAction, let's suppose it's a
fine default.

so any clonable entity relying on the copy form won't have the 'swallow copy'
warning message.

and test it.

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

the original intent was to avoid having an automatic 'clone' action appearing in
the UI because some type was adaptable to IClonable. While this is still
desirable, keeping some generic UI tweaks useful to those activating the action
will be better then dropping everything at once.

Being adaptable to IClonable doesn't mean we want a clone action in the UI,
furthermore when it leads by default to the clumsy 'copy' form.

This is information that will need most custom adapters, without doing some more
heavyweight refactorings.

For instance, following non-composite relations.

Closes #15770380.

One might want to registered different IClonable adapter (for different
selection context), and we only need to make sure that registered clone
relation information (rtype, role) are all consistent.

Otherwise we sometimes get cache issue, in particular with assertCountEqual
strange comparison algorithm.

The latter do not work with CubicWeb 3.23.

The latter does not work with Python 3.

as it may occurs during migration.

Closes #13171946

without risk :/

Closes #7166911.

So that it can be used in modules not automatically reloaded without problems.

Closes #6304702.

beside performance and consistency purpose, it will avoid to call recursivly the
clone hook if the cloned subtree contains 'clone rtype' relations. This has
caused infinite recursion problem in a client application, though I've not been
able to reproduce it in a test.