build_dependencies

        High-level api: Briefly compile all yang files and find out dependency
        infomation of all modules.

        Returns
        -------

        None
            Nothing returns.

def build_dependencies(self):
        '''build_dependencies

        High-level api: Briefly compile all yang files and find out dependency
        infomation of all modules.

        Returns
        -------

        None
            Nothing returns.
        '''

        cmd_list = ['pyang', '--plugindir', self.pyang_plugins]
        cmd_list += ['-p', self.dir_yang]
        cmd_list += ['-f', 'pyimport']
        cmd_list += [self.dir_yang + '/*.yang']
        logger.info('Building dependencies: {}'.format(' '.join(cmd_list)))
        p = Popen(' '.join(cmd_list), shell=True, stdout=PIPE, stderr=PIPE)
        stdout, stderr = p.communicate()
        logger.info('pyang return code is {}'.format(p.returncode))
        logger.debug(stderr.decode())
        parser = etree.XMLParser(remove_blank_text=True)
        self.dependencies = etree.XML(stdout.decode(), parser)