[migractions] add sanity check in rename_relation_type
If the relation is still present in the fs schema, we probably shouldn't
be renaming it (because it's still in use somewhere, possibly in a
different cube). Closes #3608172
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html