Fix for nuget v2 responses that don't specify a base

[ppejovic]

Some v2 nuget endpoints return no base attribute in the service node of the response, such as Inedo's ProGet like so:

<?xml version="1.0" encoding="utf-8"?>
<service xmlns="http://www.w3.org/2007/app">
    <workspace>
        <title xmlns="http://www.w3.org/2005/Atom">Default</title>
        <collection href="Packages">
            <title xmlns="http://www.w3.org/2005/Atom">Packages</title>
        </collection>
    </workspace>
</service>

Dependabot's current behavior results in such sources not being added to the list of dependency_urls that are probed for updates.

This fix uses the repo_details url in the case where a base url is not returned by the v2 api.

Further conversation with @feelepxyz here: c54d31c

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within seven days. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within seven days. Thank you for your contributions.

[ppejovic]

Could we get this reopened @feelepxyz? It's still a valid bug for us.

[jurre]

Pulled this in via #2518

Thanks @ppejovic!