cubicweb #1561806 RQL constraints variable namespaces [resolved]

As far as I understood a recent bug in my application, RQL constraints on a single relation are concatenated, so a variable name should not be reused (unless they represent the same entities, like "S" and "O"). It should be either clearly documented or fixed.

done in3.12.0
load left0.000
closed by#5e2f93b88d86 [entity vocabulary] refactor unrelated rql to allow usage of RQLRewriter to insert schema constraints. Closes #1561806