perl(MongoDB) is now available on all arches.
    
    Also, reverse the sense of the main package and the singular subpackage,
    so that users who install "polymake" get a working package.  The singular
    subpackage, perversely, has no dependency on Singular.  It is to be used
    to build Singular; otherwise, the Singular BR on polymake pulls in the old
    version of  Singular, which is often broken due to an soname bump.