Take the list of arches for -build Koji tag from buildrequired modules.
    
    Currently, we are using just `conf.arches` and `conf.base_module_arches`
    to define the list of arches for which the RPMs in a submitted module are
    built. This is not enough, because RCM needs to generate modules based
    on the base modules which should use different arches.
    
    This commit changes the MBS to take the list of arches from the buildrequired
    module build. It checks the buildrequires for "privileged" module or base
    module and if it finds such module, it queries the Koji to find out the list
    of arches to set for the module.
    
    The "privileged" module is a module which can override base module arches
    or disttag. Previously, these modules have been defined by
    `allowed_disttag_marking_module_names` config option. In this commit,
    this has been renamed to `allowed_privileged_module_names`.
    
    The list of arches are stored per module build in new table represented
    by ModuleArch class and are m:n mapped to ModuleBuild.