Attempt to reproduce bug

Steps to reproduce:

1. yarn install after cloning this project.

2. yarn package (or sls package) should work fine and produce two zip files (one for each function) inside .serverless.

However, if you update serverless version to v1.18.0 and do yarn package again, it will now return an error.

Serverless: Packaging service...
TypeError: Path must be a string. Received undefined
    at assertPath (path.js:7:11)
    at Object.basename (path.js:1355:5)
    at functionNames.forEach.name (/Users/noel/Documents/workspace/sls-webpack-bug-120-repro/node_modules/serverless-webpack/lib/cleanup.js:27:28)
    at Array.forEach (native)
    at fse.copy (/Users/noel/Documents/workspace/sls-webpack-bug-120-repro/node_modules/serverless-webpack/lib/cleanup.js:23:31)
    at doneOne (/Users/noel/Documents/workspace/sls-webpack-bug-120-repro/node_modules/fs-extra/lib/copy/ncp.js:237:40)
    at /Users/noel/Documents/workspace/sls-webpack-bug-120-repro/node_modules/fs-extra/lib/copy/ncp.js:122:11
    at /Users/noel/Documents/workspace/sls-webpack-bug-120-repro/node_modules/graceful-fs/polyfills.js:239:18
    at FSReqWrap.oncomplete (fs.js:123:15)