Show expanded buildrequires in API output
    
    ModuleBuild.json now includes xmd/mbs/buildrequires in the output JSON
    data. As a result, these APIs will show buildrequires by default.
    
    * /module-builds/
    * /module-builds/?verbose=True
    * /module-builds/$build_id
    
    Buildrequires is accessible like this:
    
        br_module_names = response_json['buildrequires'].keys()
        br_module_dict = response_json['buildrequires'][module name]
    
    Fixes: FACTORY-2201
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>