Avoid copying each package from the wrkdir to the repository individually
This depends on a change to pkg to makes sure not to overwrite the pkg in the repository unless it completes creating the package without error
This unlocks pkg files being named with a hash of their content (unpredictable filename), which will allow use of a CDN for pkg downloads
Related changes to pkg: https://github.com/freebsd/pkg/pull/1829
Related changes to poudriere: https://github.com/freebsd/poudriere/pull/751