Relative image paths are incorrectly allowed as example image prop values in SDCs and code component example values

Overview

It's currently possible to create a JavaScript component config entity with an invalid example image prop value by using a relative path. An error happens later when the component is being loaded:

Default images for Javascript Components must be a fully-qualified URL with both scheme and host.

component.yml shape to reproduce the issue:

name: Image URL repro
machineName: image_url_repro
props:
  properties:
    image:
      type: object
      $ref: json-schema-definitions://canvas.module/image
      title: Image
      examples:
        - src: ./hero.jpg
          alt: Example image

Proposed resolution

Move JavaScript specific checks to JavaScriptComponent::checkRequirements() which is called during discovery. This delegates to ComponentMetadataRequirementsChecker::check() for the unified checks, but can also do additional checks for code components - this image src check, the component name check, and then we can add #3587211: Empty strings are incorrectly allowed as example string prop values in JavaScript component config schema here too.

#6 mentions that SDCs are affected, but SDCs allow relative images by design, because they are considered relative to the SDC itself. We even have explicit code to handle this in SingleDirectoryComponent::rewriteExampleUrl(). I don't think we can or should break this feature, SDCs can continue to provide local images.

User interface changes

n/a