Change extract_repodata return type to namedtuple or dictionary

[SandakovMM]

Using just a tuple was fine when we only needed a few fields from the repodata. However, I would like to include the 'gpgkey' field, and we may need more fields in the future. Therefore, it would be great to change the type so we don't have to guess the order of the return fields.

There are several places where we use this function, so we should be cautious when changing its behavior.