copy a smaller subset of the tree when building repositories

We currently copy the entire repository group (as defined in the config file) before building the repository.

We could, based on the version number (and the current package copying logic, or modified if we want to change our repository layout) build a list of repositories that we want to sync, and only copy those repositories.

This is a change that could impact a large portion of curator, but isn't terribly complicated. I'd like to block it on MAKE-62.