Import modules found in the local repositories to MBS database for Offline local builds.
    
    This is the first PR in many for Offline local builds. This PR:
    
    - Adds --offline flag to build_module_locally mbs-manager command to enable
      offline local builds.
    - If this flag is used, new `import_builds_from_local_dnf_repos` method is
      called which uses DNF API to get all the available installable modulemd
      files and imports each module into MBS local SQLite database.
    - It also adds fake "platform:stream" module based on the /etc/os-release,
      so the buildrequirements of the imported modules are satisfied.
    
    The idea here is that in upcoming commits, I will create LocalResolver
    which will be similar to DBResolver with some extra rules to resolve
    local module builds. This new LocalResolver will still be based on
    the models.ModuleBuild methods and therefore we need the modules
    imported in database.