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.

sql('DROP FUNCTION IF EXISTS _fsopen(bytea)')
sql('DROP FUNCTION IF EXISTS fspath(bigint, text, text)')