Adds better support for node-gyp

[arcanis]

Some packages (for example sharp) don't list node-gyp in their dependencies even though they need it to build themselves. The reasoning they invoke is that node-gyp only is needed for some users. I don't agree with this assessment (if only because I'm running a regular OSX install and it doesn't seem to have prebuilt packages), but that's not a battle I want to start right now 🙂

Anyway, the previous fix was to manually edit the yarn.lock in order to manually add a dependency to node-gyp to the sharp package entry. It worked just fine, but was a bit hard to find for people not well versed in Yarn's internals - plus you'd have to do it every time you upgrade sharp.

Simultaneously, I didn't want to automatically install node-gyp when it is used, or use the global one. These patterns are non-deterministic, dangerous, and go against the principle of a lockfile.

This diff is a compromise: instead of crashing and letting the user find what to do by themselves, we now print an helpful error message. Additionally, we allow node-gyp to be executed from the root of the project - meaning that users only have to run yarn add node-gyp at the root of the project for everything to be built as expected.