Move some functions from paths.js into react-dev-utils/PathUtils

[ForbesLindesay]

[see #2442]

This removes the path resolution utilities from config/paths.js and
leaves behind just the actual path configuration.

[gaearon]

Looks like CI is failing:

+npm run build
> @ build /home/travis/build/facebookincubator/create-react-app
> node packages/react-scripts/scripts/build.js
module.js:471
    throw err;
    ^
Error: Cannot find module '/home/travis/build/facebookincubator/create-react-app/packages/package.json'
    at Function.Module._resolveFilename (module.js:469:15)
    at Function.Module._load (module.js:417:25)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at getPublicUrl (/home/travis/build/facebookincubator/create-react-app/packages/react-dev-utils/PathUtils.js:35:19)
    at Object.<anonymous> (/home/travis/build/facebookincubator/create-react-app/packages/react-scripts/config/paths.js:75:16)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)

[stale]

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