clobber-registry: fix pypy clobber issue

[wolfv]

Currently, when switching from cpython to pypy, there is an interesting clobber case:

For cpython, the python package contains bin/python.

When installing python 3.9.* *pypy*, the bin/python package is in the pypy38 package. However, the python package also still exists, (but with a different build string and without the bin/python inside. Because python depends on pypy38, the "python" package wins the clobbering. But no bin/python is created, only bin/python__clobber-from-pypy38...

I will have to think of how to fix this case.