cubicweb-container #4197401 use plain rdefs everywhere [done]

This is an old nagging design flaw: we use etypes and rtypes sets to shape the container.
When people start playing with rtypes that have rdefs in the container, at the border, or in outer space, things fall a bit apart.

priorityimportant
typeenhancement
done in2.7.0
load0.000
load left0.000
closed by#8f804ee46af4 Switch to rdefs to define the container structure, and make the container relation reflexive
patchConfig: automate the registration of the clone adapter [applied]Deprecate the old config rtype-based API [rejected]Switch to rdefs to define the container structure, and make the container relation reflexive [applied][config] Introduce container rdefs computation [applied]
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