pkg: clean support for repositories

Rework the way the bootstrap fetches pkg, by implementing a full support
for the repositories, the boostrap will now loop over all available repo
and try to fetch the full package from there. It will at the first valid
package found.

Fallback to packagesite (which has been deprecated for a while) if needed, by
transforming it into a repo, if no repo is found.

MFC After: 3 weeks

(cherry picked from commit dc4581589a3256667fafd46a30c67abdfd86618f)