Protect against crash in the `relation_possible` predicate with ambiguous relations.
It was still possible to get a KeyError when using relations that are ambiguous
at both ends, like (Executable, version_of, Program) & (Version, version_of, Project).
Closes #3010148.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared