relation_schema_for and entity_schema_for replace rschema and eschema.

This folder is generated by yapps from the grammar.
No need to check flake8 here.

fix(attribute or function): Allows to apply function or get attributes on projection variable with eid relation

This allows to query something like "Any X.attribute WHERE X eid 42"
The query was simplified using the syntax "Any 42" and the attribute could not be reached.

type:ignore comments are used to silence mypy errors
let's remove the ones that are not anymore necessary

related: cubicweb#528

These types are not supposed to be accessed by client code.