fix: Do not rollup vendor imports into package

Prevent large libraries like echarts from being included in the rollup output. Instead let the library's consumer handle their inclusion.

Before:
(from webpack-bundle-analyzer in gitlab-ce)

After:

Note: this won't, by itself, reduce the bundle size within gitlab-ce, but it will show more transparently what is taking up space in the bundles by not obfuscating gitlab-ui's dependencies within one giant index.js blob.