package: Mark visit as not_found when relevant

If any errors happen during the communication with the origin to retrieve package
information, the visit fails and its status is marked as not_found.

Only pypi, npm and nixguix package loaders are impacted. The other loaders do not read
anything from their url so there is no way to trigger such possibility.

Note that the nixguix loader got refactored to avoid the side-effect of reading data out
of the url within the constructor. It was necessary so the check fails and the visit
status is dealt with as described. Also, this unifies it with how pypi and npm loaders
deals with the url communication.

Related to T3030