- A single primary repo must be specified which
holds the site_definition.yaml file.
- Zero or more auxiliary repos can be specified which
have additional documents used in the site definition.
- Collected documents are written to a file named after their
source repo. Collection must always be given a output directory
rather than a single file now.
Change-Id: Iceda4da18c4df45d917d88a49144e39e3f1743ed