Add module metadata for modules downloaded from Koji
    
    As of Fedora 29, dnf enforces module dependencies, so to install an
    application module that depends on flatpak-runtime, the DNF
    repository with flatpak-runtime content needs the modulemd included.
    However, MBS doesn't add the modulemd when downloading module
    content from koji. So we create a second directory, symlink the
    MBS-downloaded content into it, and run createrepo/modifyrepo again.