I can understand finding packages via the implicit (“flat”) layout by default, as well as in a src directly (which I personally strongly prefer for its explicitness and avoiding all the heuristics added here, as well as the various other issues), but does finding namespace packages really need to be turned on by default. At least in my understanding, their use is not that common, at least intentionally, are sometimes (mis)used without even knowing it, and this seems to be as likely to run into edge cases like this one where this behavior is assumed but not intended as those where it was.
Given that, is it possible to have find.namespaces default to False (at least initially), and those who actually need it can simply flip it on? Explicit over implict, after all.