[Catalog-sig] PyPI naming policy changes

["Martin v. Löwis"]

> Probably the test you want here is:
> 
>    safe_name(filename).startswith(safe_name(project_name))

So what's to_filename then for? Also, is it allowed for the
distribution's name to be all-lowercase, when the PyPI package name
is mixed-case?

In any case, I updated the formula in r543.

Regards,
Martin