cubicweb #1973767 ms planner should raise BadRQLQuery when specified sources for some variabes are incoherent [resolved]

instead of generating a bad plan

prioritynormal
typebug
done in3.13.7
load0.200
load left0.000
closed by#b1c913a6d9f0 [ms planning] raise BadRQLQuery when two variables related by a non-crossable relation should come from two different sources (closes #1973767)