cubicweb #3789914 [shell] replace usage of global variables with a cw namespace [open]

Instead of using global variables ("session", "config", etc) in the cubicweb shell, we could define a cw namespace for those ("cw.session", "cw.config"). This would avoid problems when one has Configuration etypes in the schema and uses (by mistake) "config" as a variable for that kind of entity.

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