adapted from uitil <https://github.com/FND/uitil>

this removes the `src` directory in the distro package in order to
simplify imports (avoiding "src" appearing on the import path) and
ensures the distro package only contains files under version control

note that we're excluding any Babel configuration, which required
relegating that to a separate file - this is important because we want
to be transpiler-independent, i.e. it's just ES2015 source code, how (or
whether) that's transpiled is up to the consumer