Dependency resolution errors for URL requirements that don't exist in PyPI

[tetsuo-cpp]

I noticed this as part of debugging #354. Context in #354 (comment).

[tetsuo-cpp]

So the reason that the example from the linked comment causes a dependency resolution failure is because the project page exists on PyPI but it has no links (as opposed to returning 404).

The link is here so you can have a look: https://pypi.org/simple/en-core-web-sm/

There doesn't seem to be a package with that name so I'm thinking that perhaps just the package name has been squatted but no versions have been published. Anyway, I think we should just treat this as not finding the project on PyPI.