Handle cross references for third party libraries

[dandhlee]

Googlers see b/196413561.

Eventually, we should be able to handle cross references that link to third party libraries.

Several questions follow: where do we extract the info for each library? Pypi might be a good place to start.
Should we direct to the package link that's hosted on GitHub? What if the package sites aren't hosted on GitHub? What if there is an external website that is a better place to cross reference to?

.NET has https://xref.docs.microsoft.com/query?uid={uid} query one can call for .NET products, perhaps Python should get one as well.