Revert the previous approach on multiple URLs and prefer a more compatible approach

Reverting the "archive" and similar fields to their previous types, and adding a "mirrors" field (that must be of the same type).

Compared to the previous approach:
* doesn't break the API, only extend it
* repo format remains backwards-compatible
Merge pull request #1094 from AltGr/more-compatible-mirrors

Revert the previous approach on multiple URLs and prefer a more compatible approach