This has proper support for rollup while providing the same API.

This is what open-wc recommends now and uses in their @open-wc/testing package

Takes a package name and a dest path. This way we can more eassily change the
layout and it will error out on package name typos or missing packages.

Use code splitting instead

also provies a function for css content icons (a bit hacky as it uses masks)

node-resolve wasn't limited to the local node_modules and picked up the
dependency from another package on my machine.

When specifying a package for code splitting, rollup requires the
packages to have a valid entry point for some reason.

Add an empty one.

So rollup doesn't have to convert them (and potentially fail)