Better support for single project garn files

[soenkehahn]

When working with garn on a project that only uses one toolchain, it's a bit inconvenient that all Checks and Executables are qualified under that project. So it's e.g. garn run myCoolProject.dev, garn check myCoolProject.eslint, etc. There should be an easy way to allow garn run dev and garn check eslint.

export default myCoolProject could be an option for a nice API.