source: When fetching, try to fetch from mirrors first

**KLUDGE WARNING**: This involves making the source store its "meta"
object so that it's possible to create a copy of the source inside the
fetch queue, instead of back when the pipeline was being loaded.

This adds the SourceFetcher class, which is intended for sources that
fetch from multiple URLs (e.g. the git source and its submodules)

Fix when fetching