Add network-based module and group files download

[Ayowel]

At the moment, module and group files are loaded from the cache, which might not work in configurations other than the ones currently tested. Adding networking support might make downloads more reliable across systems that use dnf. We should be able to:

• download data from remote locations
• enforce cache-based and network-based downloads with an option

Note that for performance, we will probably need to restructure some commands so as to allow repomd caching during network-based downloads