Make sources rendering easier

Problem/Motivation

In the UI Patterns ecosystem, including Display builder, there is a recurrent needs of rendering any single source, or a list of source. Because the source tree has not always a SourceComponent as a single root node.

So, the community do weird tricks like:

    foreach ($sources as $source_data) {
      $fake_build = $this->componentElementBuilder->buildSource($fake_build, 'content', [], $source_data, $contexts);
    }
    $build = $fake_build['#slots']['content'] ?? [];
    $build['#cache'] = $fake_build['#cache'] ?? [];
    // The render array is built based on decisions made by SourceStorage
    // plugins, and therefore it needs to depend on the accumulated
    // cacheability of those decisions.
    $cacheability->applyTo($build);

where a single public function buildSources(array $sources, array $contexts = []): array may be enough.

Proposed resolutions

1. Add a SourceTreeRendererInterface with a ::buildSources() method
2. Implement it:
   • in ComponentElementBuilder: to keep things simple. SourceTreeRendererInterface namespace can become a service alias. ComponentElementBuilder::buildSource() can be deprecated and become protected or private later.
   • OR in a brand new service, using SourceTreeRendererInterface namespace as service ID. This will allow a cleaner implementation and to also add SourceTreeRendererInterface::buildSource() later. ComponentElementBuilder could become @internal or deprecarde.
3. Use SourceTreeRendererInterface::buildSources() everywhere a "fake component' is used:
   • modules/ui_patterns_field/src/Plugin/Field/FieldFormatter/SourceFormatter.php
   • modules/ui_patterns_field_formatters/src/Plugin/Field/FieldFormatter/SourceFormatter.php
   • modules/custom/ui_patterns/src/Plugin/UiPatterns/Source/LayoutSource.php

The implementation in ComponentElementBuilder can be naive and keep the "fake component" mechanism (at leas, it is centralized in a single place now):

  /**
   * {@inheritdoc}
   */
  public function buildSources(array $sources, array $contexts = []): array {
    $cacheability = new CacheableMetadata();
    $fake_build = [];

    foreach ($sources as $source_data) {
      $fake_build = $this->buildSource($fake_build, 'content', [], $source_data, $contexts);
    }
    $build = $fake_build['#slots']['content'] ?? [];
    $build['#cache'] = $fake_build['#cache'] ?? [];
    // The render array is built based on decisions made by SourceStorage
    // plugins, and therefore it needs to depend on the accumulated
    // cacheability of those decisions.
    $cacheability->applyTo($build);

    return $build;
  }

Or being more ambitious, like the implementation in a new service would be.

Remaining tasks

Follow-up: in Display Builder, use SourceTreeRendererInterface::buildSources() in:

• EntityViewDisplayTrait
• DisplayBuilderPageVariant
• PreprocessViewsView
• BuilderPanel
• PreviewPanel
• Maybe more...