In Node, we now use a CommonJS build with external dependencies, rather than
inlining the dependencies as is done with the UMD bundle. Thus, Node now
automatically defers to the dependent’s main entry; in the case of d3-request,
that means it inherits the XMLHttpRequest polyfill.

A slight wrinkle is that Rollup doesn’t correctly re-export the d3.event symbol
if the dependencies are not inlined. However, this is fixed by patching the
CommonJS build and replacing the event export with a getter.

This commit also removes the rollup-plugin-json dependency, and instead
generates a tiny ES6 module to export the version field from package.json.