PRO-6835: Determine Best AssetUrl is in easy mode with the new build system

[myovchev]

Summary

The new build system (when external build module is self-registered) doesn't need any computations about "what is the override of a given module". The asset override is done on fs level and the path doesn't need "my-*" prefix in the URL. However, the legacy Webpack system still needs the computations. This patch fixes it all in a BC way.

What are the specific steps to test this change?

Override a preview image for @apostrophecms/image-widget in modules/@apostrophecms/image-widget/public/preview.jpg (project level).

When no external build module (Vite) is registered the preview will be resolved (and properly shown) to:

/apos-frontend/default/modules/@apostrophecms/my-image-widget/preview.jpg

When external build module (Vite) IS registered the preview will be resolved (and properly shown) to:

/apos-frontend/default/modules/@apostrophecms/image-widget/preview.jpg

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-6835 Preview images broken for widgets with project level overrides