Slightly rework the load_fedora_remote code

This is a trivial change, to make the next one easier to review.

Yes, the "if True" is silly, but I'm going to have something in the next
commit, I don't want to keep reindenting that block.
fetchfedora: Take renamed packages into account

We sometimes rename a package. For example, "nbboost" is in fact "boost"
from Fedora, that we renamed (and relocated in /opt) because we already
have a "boost" package in NBRS 5, the EL version, which we'd rather not
upgrade to the Fedora version.
fetchfedora: Be more helpful on errors

Most likely, if this is failing, it's because the user forgot the --name
argument and the module has been renamed.

So let's say that.
sources: Restore the previous lookaside

Using nbpkg as an API would cause problem without it, for example when
trying to get the Fedora sources, then ours.
sources: Take renamed packages into account

This is similar to what we did for the fetchfedora command.