RFE: add module metadata

[ausil]

We should be providing a module.json file in the compose metadata that has info on the modules included in a compose. similiar to rpms.json and images.json

[dmach]

+1 to this feature.
Unfortunately there are several issues that should be addressed first:

• modulemd doesn't support $arch
• there's no unique identifier for modules yet; I've opened a request in modularity trello for that couple months ago, proposing $name-$stream-$version.$arch[.module].yaml

I'll be pushing on both issues as DNF requires them too.