npm link causes sam local to error

Hello! I'm using sam local to rapidly develop and test nodejs lambda functions.

SAM version: SAM CLI, version 0.6.1
MacOS 10.13.6

I have common code in separate node modules that I am trying to use 'npm link' with to create symlinks for in my lambda functions node_modules directory. This would be a great way to allow editing the common code in directories outside of my lambda function directories and just have those changes immediately available to my lambda functions where they are linked in the lamdba function node_modules directories. Otherwise, every time I change the common code I would have to go through extra steps to copy it to every lambda functions node_modules directory for the lambdas that need to use that common code.

The advantages of this is explained very well here:
https://forums.aws.amazon.com/thread.jspa?threadID=214756#jive-message-671727
and here:
https://stackoverflow.com/questions/33511652/including-local-dependencies-in-deployment-to-lambda

I can see that my symlinks are created properly and I can 'ls' the symlink to see that it lists the expected files back in the linked directory. However, when I run my code and try to test when using these symlinks I get:

```
START RequestId: f453f83b-58e5-1a7f-acd9-1af0f9a251c5 Version: $LATEST
Unable to import module 'index': Error
at Function.Module._resolveFilename (module.js:547:15)
at Function.Module._load (module.js:474:25)
at Module.require (module.js:596:17)
at require (internal/module.js:11:18)
at Object. (/var/task/index.js:3:13)
at Module._compile (module.js:652:30)
at Object.Module._extensions..js (module.js:663:10)
at Module.load (module.js:565:32)
at tryModuleLoad (module.js:505:12)
at Function.Module._load (module.js:497:3)
END RequestId: f453f83b-58e5-1a7f-acd9-1af0f9a251c5
```

If I copy my common node modules directly into my lambda function node_modules directories everything works perfectly.

Any suggestions? Does it sound like I am doing something wrong? Is this a known issue?
If I can copy my node module directly into my lamda node_modules directory rather than using a symlink it seems like there might be some issue preventing this. Having this capability would greatly help developing node lambda functions to be much faster.

Thanks!