Refine the warning displayed when node_modules is detected in the application directory

[colincasey]

When the nodejs-npm-install buildpack runs it's build process, it checks for the presence of a node_modules folder in the application directory and emits a warning if the folder is found.

This warning makes sense when the buildpack executes on Heroku since a build is triggered by a Git push and that would indicate the user has checked in node_modules into their source control. But, when running pack locally, this warning is not required. The check should be refined to:

• not emit the warning if the application directory is not a Git repository
• not emit the warning if the user already has node_modules listed in their .gitignore