fix(build): ensure dist file name is correctly inferred

[LinusBorg]

When building a package with a scoped name like:

{
  "name": "@linusborg/my-cool-lib"
}

this will create the following folder structure:

dist/
  |- @linusborg/
    |- my-cool-lib.es.js

But what we want is:

dist/
  |- my-cool-lib.es.js

This PR solves this by stripping the package name's scope before using it as the output filename for rollup.