Support for scoped package names

[joshwlewis]

If a function's package.json name includes a slash, as is common for scoped npm packages, we'll fail to create the symlink.

The error looks like this:

failed to create symbolic link '/layers/salesforce_nodejs-fn/middleware/node_modules/@elbandito/square': No such file or directory

This is likely because /layers/salesforce_nodejs-fn/middleware/node_modules/@elbandito does not exist, so we cant put a symlink in it.