fix(client): entrypoint imports would race

[LeoMcA]

Noticed while fixing #1367.

If a hook imported from the client entry point would await at the top level, we'd skip over it and go and continue executing some other scripts.

That breaks an assumption I had held, and breaks l10n loading.

There may be some cases where we're ok with things racing: like scrolling to the current item in the sidebar while we load elements.

Opened as draft to see how bad this feels on a remote environment.

[github-actions]

ebe633a was deployed to: https://fred-pr1368.review.mdn.allizom.net/