Define new performance_bar entrypoint properly

With https://gitlab.com/gitlab-org/gitlab/-/merge_requests/151025 we
introduced a new entrypoint for the performance bar. Unfortunately we
didn't end up using it, because the webpack config pointed to
`~/performance_bar/index.js` instead of
`~/entrypoints/performance_bar.js`.

This resulted in the performance bar being broken in GitLab instances
with relative root installations.

Changelog: fixed