cubicweb #506878 rename schema/ and entities/ [open]

Strictly speaking, the schema is the drawing of the data model. We could rename the schema/ directory to model/. And the entities/ directory contains the logic/ of the application. That would get us to model/, logic/ and views/, which is even closer to something understandable without documentation. Maybe the contents of hooks/ could be moved to logic/ at the same time.

Of course the old directories would be supported and warnings issued for the next two versions.

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