The repos format loaded by images is different from rpmmd.RepoConfig. The on-disk format has `BaseURL string` while RepoConfig has `BaseURLs []string`. When using image-builder to generate manifests in osbuild-composer, we need to write a repos file to disk for image-builder to pick up. To do this, we need to write it in a format that IB can load (using rpmmd from images).

We should make the repo config loader also support BaseURLs []string. For backwards compatibility, we should keep BaseURL string as well. Both should be allowed and for simplicity, we can merge them (append BaseURL to BaseURLs).

This is the same as what we did for GPGKeys.