gather: Honor module whitelist
    
    If the modulemd contains a whitelist of packages (under buildopts), it
    means the packages are possibly renamed, and we need to check that list
    instead of components.rpms.
    
    Multilib does not really work now, anything with non-native arch is
    skipped.
    
    JIRA: RCM-38019
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>