bug#35673: pypi importer returns the wheel hash when it fails to find a proper source.

[Jack Hill]

As discussed on guix-devel[0] the pypi importer should probably not fill 
in the hash with that of the wheel if a proper source archive is not found 
on pypi.

[0] https://lists.gnu.org/archive/html/guix-devel/2019-05/msg00209.html
Best,
Jack