feature request: pre-fetch all sources

Prevent a scenario where overnight builds fail because of network or 404 issues. Find these early.

This pre-fetch mechanism could even (externally) support retries without having to modify abuild-fetch.

Also with the recent changes to force a clean environment during system/ build, build (or fetch) failures are not tolerated at all.

Since system/ does not change often, we should be able to assume that having all sources ahead of time will result in a successful build.

Instructions/documentation need to be written for how to recover from any failure during the system/ first-pass build.

The quick hack is rm -fr /.autobuilder/*/rootfs/ which DOES NOT remove any packages or keys etc.