Mixing the way to build a distribution with where to get the source
packages from is wrong: it breaks pre-release repos, local mirrors, and
other use cases. To accommodate those, we introduced
`/etc/osbuild-composer/repositories`.
However, that doesn't work for the RCM API, which receives repository
URLs to use from outside requests. This API has been wrongly using the
`additionalRepos` parameter to inject those repos. That's broken,
because the resulting manifests contained both the installed repos and
the repos from the request.
To fix this, stop exposing repositories from the distros, but require
passing them on every call to `Manifest()`. This makes `additionalRepos`
redundant.
Fixes#341
77556973cc