Support Symbolic Links in "vendor" directory

[TimLudwinski]

Currently if the vendor directory contains symbolic links, each of those links will contain a copy of the target file inside the output zipfile. If the original file was large, this can cause the lambda to go over the size limit.

Here is a python example of creating a zip entry with a symbolic link: https://stackoverflow.com/a/41573364/1413201. This likely can be adapted for nodejs.