Attempt to remove node_modules when it's not emtpy

[mars-lan]

During the clean step, the plugin assumes that it's okay to remove the entire node_modules if there's only scoped packages left: https://github.com/Butterwire/serverless-plugin-monorepo/blob/master/src/index.ts#L143

I'm not sure the logic behind that, but if you happen to have a node_modules with only non-hoisted scoped packages, it'll lead to the following error

Serverless: Cleaning dependency symlinks
 
  Error --------------------------------------------------
 
  Error: ENOTEMPTY: directory not empty, rmdir '<package_root>/node_modules'

[avnerenv0]

Also happening to us :(

[martin-css]

A check has now been added to ensure the directory is truly empty before deleting.