There is an issue where you can interrupt a gatsby project build and the temporary files are not cleaned up. If you try to build again, it will get stuck in an infinite loop.
This PR adds an exit hook to make sure the cleanup happens.
Right now, static-build will add the necessary Gatsby plugins to the project's `package.json` at build-time, which has been bothersome for package managers when using a frozen lockfile.
Another issue with it is that we install `latest` version of the plugin, so the version used becomes disjointed from the CLI version itself, which leads to unpredictability when trying to debug issues or help users roll back to a previous behavior if something breaks.
So instead of patching `package.json` directly, include the plugins as deps of static-build itself, and create symlinks to those paths into the project's `node_modules` directory.
Adds some unit tests for the gatsby injection logic.
These are net new since we've heavily changed the injection logic. The tests and fixtures in `build-fixtures` and `builds.test.js` are seemingly not executed. We may delete those here too.
