def set_package_name(self, package_name):
<EOL>self._package_name = package_name<EOL>root_module = self._import(package_name)<EOL>self.root_path = root_module.__path__[-<NUM_LIT:1>]<EOL>self.written_modules = None<EOL>
Set package_name

        >>> docwriter = ApiDocWriter('sphinx')
        >>> import sphinx
        >>> docwriter.root_path == sphinx.__path__[0]
        True
        >>> docwriter.package_name = 'docutils'
        >>> import docutils
        >>> docwriter.root_path == docutils.__path__[0]
        True