Fix for #3908

[jaiminpanchal27]

Type of change

• Build related changes

Description of change

Fix for #3908
Recently many use cases have come across where we needed submodules for prebid. Example include

• user id module and various id systems
• long form module and supported long form ad servers
  Also in future we would like to add other features as modules and not part of core.

Until we have a new build system and fresh approach to modules this PR fixes #3908.

This fix returns the modules which are included in module.json or as command line arguments to gulp.src.

Command 1: gulp build --modules=appnexusBidAdapter,digiTrustIdSystem --analyze
Result

Command 1: gulp build --analyze
Result

[snapwich]

you're putting the fix in the wrong place; we need to fix the webpack process. this partially fixes it: #3924