Fix debian repo creation

This commit addresses an issue where the debian package repositories created
for jenkins package builds clobber each other for each new dist. This commit
creates the originally intended behavior, which was to create a repository
for each distribution individually. It does so by creating the repositories
under $project/$sha/repos/apt/$dist whereas before they were all just in
$project/$sha/repos/apt. It also adds better error handling around the case
where we attempt to create debian repo configs when there are no repos to
create configs from.