fedmod currently only loads the repository metadata for the F27 Modular Server composes. This isn't right, since the Modular Server compose only includes a subset of all of the software available in F27.

So the repo config needs to be updated to separate two things:

1. The repos that will be used to determine which modules provide which RPMs (these will be the modular server repos, as they're the only ones that currently provide module metadata)
2. The repos that will be used to calculate the repoclosure for the given packages (these need to provide metadata for the full Fedora package set)

(Note: this is a distinct issue from https://pagure.io/modularity/fedmod/issue/4, as that's about providing runtime configurability, whereas this is about making the existing F27-only hardcoded config work properly. Implementing this will help clarify what runtime config info is actually needed)