Create symlinks on function deployments

[martin-css]

It is common when developing lambda functions to deploy these individually after making changes, e.g. sls deploy -f myfunc.

This causes problems when the node_modules directory has been cleaned, e.g. by yarn install. This will cause no dependencies to be uploaded, creating an invalid package.

Modify the plugin to listen for the appropriate hook so that it will create relevant symlinks when using both sls deploy & sls deploy -f.