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.

as it may occurs during migration.

without risk :/

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.

This is not ready for inclusion and was backported from client
application to soon.

Thus grouping related things together.

Otherwise the assertion in __registered__() is hit at db-init step.

This method returns a dictionary mapping entity type to composite relation
information allowing to walk the schema graph upstream from these entity
types.

* `parent_relations`/`child_relations` are used to get relation information
  from an entity type.
* `parent_structure`/`child_structure` return the full structure of the
  composite graph, this structure will change later on to conform to the
  "definition" methods.
* `parent_related`/`child_related` yield information about related entities in
  the composite graph either from the parent or child perspective.

These are used to build the IContainer/IContained classes.

Properly walk through relation definitions accounting for parent role and
entity type in iter_schema. Better use Yams API in passing.

This makes selections of CloneEntityHook simpler, cheaper and more robust.
Drop IClonableAdapter.clone property, not longer useful.

This is not used currently.

The dictionary was not really useful, a set is enough. Also sort these
children in `structure()` method for predictability.

Also add the `structure` method which returns the fully evaluated
graph (of schema objects), useful for testing.

And pass them to composite_schema_graph.

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