hook_requirements uses a renderable array, but renderable array only supported during runtime

Problem/Motivation

image_widget_crop_requirements() implements a renderable array to display errors:

if (!empty($error)) {
    $requirements['iwc_libraries']['severity'] = REQUIREMENT_ERROR;
    $requirements['iwc_libraries']['description'][] = [
      '#theme' => 'item_list',
      '#items' => $error,
    ];
  }

However, renderable arrays are only supported during runtime:

hook_requirements() messages currently work with render arrays for runtime requirements messages. However, the documentation does not explicitly state that render arrays are allowed. Additionally, install phase requirements do not support render arrays because in drupal_check_module()they are passed to drupal_set_message(), and also to a placeholder in a t() following #2501639: Remove SafeMarkup::set in drupal_check_module().

#2505499: Explicitly support render arrays in hook_requirements()

Proposed resolution

Only return a renderable array in hook_requirements during the 'runtime' phase.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.