Motivation

Fix #11430

For some reason, the following code still tries to resolve/ bundle @mermaid-js/layout-elk even when __DOCUSAURUS_MERMAID_LAYOUT_ELK_ENABLED__ is false (in which case the bundler is supposed to ignore/remove that code).

  if (__DOCUSAURUS_MERMAID_LAYOUT_ELK_ENABLED__) {
    const elkLayout = (await import('@mermaid-js/layout-elk')).default;
    mermaid.registerLayoutLoaders(elkLayout);
  }

This bug only happens with the following conditions:

• in the server config (not the client one)
• when using dynamic imports (and not require() calls)
• when using the SWC loader, Rspack built-in or not (and not when using Babel loader)
• when using Rspack (and not when using Webpack + SWC)

I tried to reproduce here in isolation, but unfortunately couldn't
https://github.com/slorber/rspack-webpack-bug-repro-template/blob/slorber/repro-definePlugin-optional-dep/test.js

Since I'm not able to reproduce, I'm going to implement a decent workaround, aliasing the non-existent package to "undefined" so that resolution never fails when the package is absent. Not ideal, but it works well.

Test Plan

CI + local tests

Test links

https://deploy-preview-11437--docusaurus-2.netlify.app/