* Unify the way we fork modules
* Replace rollup-plugin-alias with our own plugin
This does exactly what we need and doesn't suffer from https://github.com/rollup/rollup-plugin-alias/issues/34.
* Move the new plugin to its own file
* Rename variable for consistency
I settled on calling them "forks" since we already have a different concept of "shims".
* Move fork config into its own file