Update navigator image references to the correct relative path

[d-ronnqvist]

Bug/issue #, if applicable: rdar://128397787

Summary

This fixes a bug in the navigator index where pages with custom @PageImage icons would use the wrong relative path to refer to that image within the .doccarchive.

Dependencies

None.

Testing

• In any project, add an article with a custom page image icon (and add an image to use as the icon). For example:

# Some article

@Metadata {
    @PageImage(purpose: icon, source: "my-icon")
}

An article with a custom page image.

• Build and preview documentation for the project.
  • The article's item in the navigator should display the custom icon

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary Not applicable

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

This asset prefix parameter configures the encoder to insert the bundle ID in the relative path for all assets.

[emilyychenn]

Looks good to me!