Then why to use fpm in the first place? If a user has full control on all the packages he’s combining (they are all in his stack or from collaborators), shouldn’t he be able to take responsibility over his actions and join them at his own risk?
So a non-conforming project could be used (but not registered), but maybe with a loud warning punched directly in the face, so to warn that this is highly discouraged behavior unless you really know what you are doing (basically self-managing a nonconforming ecostystem).
I mean there is nothing worse here than the usual CMake projects, where there is no enforcing whatsoever on module names and clashes could happen anytime. So if CMake allows for brewing dangerous combinations and fpm does not, in any way, people that do not want to migrate to the new established convention would go the full outcast way, and never change the build system. If you allow them, instead, to keep doing their stuff with just the work needed to change build system (which already can be nontrivial, adn that’s unavoidable), they could gradually fall in love with the new improved workflows and at some point evaluate going to full conformance, for their own benefit (that they had a taste to).