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

When the `nodejs-npm-install` buildpack runs it's build process, it [checks for the presence of a `node_modules` folder](https://github.com/heroku/buildpacks-nodejs/blob/main/buildpacks/nodejs-npm-install/src/main.rs#L107) 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`