Step to reproduce

Install dep

yarn

Build package and check artifacts

yarn sls package
cd .serverless/artifacts && unzip *.zip
Archive:  bug-hello-1536572322011.zip
  inflating: functions/handler.js    
  inflating: functions/handler.js.map  
  inflating: node_modules/test/package.json  => test is here
  inflating: node_modules/test/index.js  

Link test folder.

cd ./test && yarn link && cd - && yarn link ./test

Check symlink (node_modules/test -> ../test)

l -d node_modules/test 

Build package

yarn sls package

Check package

cd .serverless/artifacts && unzip *.zip
Archive:  bug-hello-1536572498098.zip
  inflating: functions/handler.js    
  inflating: functions/handler.js.map  
==> no test folder

=> As you can see test modules is not on the bundle.