Make hashes optional (#910)
There is no guarantee that indexes provide hashes at all or the sha256
we support specifically. [PEP
503](https://peps.python.org/pep-0503/#specification):
> The URL SHOULD include a hash in the form of a URL fragment with the
following syntax: #<hashname>=<hashvalue>, where <hashname> is the
lowercase name of the hash function (such as sha256) and <hashvalue> is
the hex encoded digest.
We instead use the url as input to generate a hash when caching.
