Evaluate relative paths against package.json

[romanzoller]

Evaluate relative paths against package.json instead of process.cwd().

This fixes an issue when using an eslint IDE plugin (e.g. in vs code),
in a project where the javascript code is not at the project root.

Example structure:

src/.eslintrc
src/package.json
src/static/app/app.js // starts with goog.module('app.app')

with googshift/valid-provide-and-module root option set to 'static'.

Using process.cwd(), the rule would expect the file to be at
static/app/app.js
instead of
src/static/app/app.js