I was able to solve below using specifying file name in assets

webapp.js
const serverType = opts.serverType || 'http';
const serverImpl = require(serverType);   -> variable inside require

package.json
"assets": "lib/server/webapp.js"

But Im stuck in below

index.js

require(TOPIC_SETTINGS[argv.topic].backend);
require(`processors/${argv.topic}/processor.${processorModule}`)(dbClient));

package.json
"assets":"index.js"

getting below error

after i taken out dynamic part of the code

const temp1 = './lib/' + TOPIC_SETTINGS[argv.topic].backend + '.js'
const DbClient = require(temp1);


Warning Cannot resolve 'temp1'
 /home/nilesh/index.js
Dynamic require may fail at run time, because the requested file
  is unknown at compilation time and not included into executable.
  Use a string literal as an argument for 'require', or leave it
  as is and specify the resolved file name in 'scripts' option.