Fix copying files to build/npm-react recursively

The intention of the `npm-react/**/*` rule was to copy recursively but it would have flattened any nested directory structure. I changed the `build/modules` rule to copy recursively as well, which is necessary for `ReactTestUtils` to be able to require `test/mock-modules.js` successfully. (`ReactTestUtils` isn't included in a clean `npm-react` build currently but it will be in the future.)

This also makes running ART tests more practical.
Merge pull request #918 from spicyj/npm-react-rec

Fix copying files to build/npm-react recursively