Add npm commands to check whether node_modules is platform-independent

[fab1an]

Build systems like gradle can cache the entire npm install process and related node_modules folder and make up to date checks using the hash of the folder. Unfortunately this is packed into a zip-file which doesn't allow symlinks and it's necessary to be careful because of platform dependent modules.

I think it would help various build systems down the way if npm could provide a flag like npm --check-platform-independance that checks whether node_modules has some platform dependent code, or can be reused freely.