Returns the location of site-packages in the python-install build
        dir.

def get_site_packages_dir(self, arch=None):
        '''Returns the location of site-packages in the python-install build
        dir.
        '''
        if self.python_recipe.name == 'python2legacy':
            return join(self.get_python_install_dir(),
                        'lib', 'python2.7', 'site-packages')
        return self.get_python_install_dir()