Rewrite %py_package, add %py_modname
    
    Clean up %py_package to get as much as possible from the main package
    declaration.
    
    Add %py_modname to extract the module name from the package name.  If
    I'm right, this can simply be overridden in the spec if the module name
    doesn't match the package name (with python- prepended).
    
    Add %py_modname_ver for convenience.