[req.find] Use vreg.schema.eschema for eschema lookup
If an etype is non-existant, using vreg.schema[etype] raises a confusing error
complaining about a non-existing relation. This is because of the
implementation of vreg.schema.__getitem__ that look first in entities and then
in relations.
Using directly vreg.schema.eschema restrict the lookup to etypes only, hence
raises a meaningful error when the etype does not exist.