Wheels built for namespace packages on older setuptools are stuck without robust support

[jaraco]

Prior to 28.3.0, the -nspkg.pth files would mask the PEP-420 Namespace loader. See pypa/pip#1924. With 28.3.0, setuptools now builds those .pth files to disable the functionality on Python 3.3 and later. However, wheels built using older setuptools will already have those .pth files generated in the wheel so will never have the workaround. As a result, all wheels for all namespace packages need to be (re-)built using setuptools 28.3.0 or later in order to include this functionality.

[jaraco]

Following the fix for #250, this issue is no longer relevant to PEP-420, but is instead relevant to simple, robust namespace package support. A wheel built with an older version of setuptools will not have the robust namespace support.

I still don't think there's anything that can be done about this issue unless we somehow design the wheel installer to re-generate these -nspkg.pth files.