Support libraries rewriting using a webpack manifest.json

This is a really good idea.
A solution I use right now is to avoid the version key in the my-module.libraries.yml files, this way we avoid having cache issues on the visitors when a library is updated (and in case we forgot to update manually the version field).
The problem with this approach is that a new version number is generated every time the cache is cleared, leading to useless libraries downloads from the visitors.

Adding a manifest field in the my-module.libraries.yml might solve that:

some-lib:
  manifest: js/manifest.json
  js:
    js/dist/my-lib.min.js: {}

This is only an example on how it could be implemented. With this field, Drupal would get the generated my-lib.min.js file (example: my-lib-abc123456.min.js) instead of the non-existing my-lib.min.js.

I recently ran into the same issue. My workaround consists of implementing hook_library_info_alter() that replaces the asset source with the destination within a defined manifest.json and looks something like this.

<?php
function my_theme_library_info_alter(array &$libraries, string $extension): void {
  /** @var \Drupal\Core\Extension\ThemeExtensionList $theme_extension_list */
  $theme_extension_list = \Drupal::service('extension.list.theme');
  $module_path = realpath($theme_extension_list->getPath('my_theme'));

  foreach ($libraries as $name => $library) {
    if (isset($library['js'])) {
      foreach ($library['js'] as $asset_source => $options) {
        if (isset($options['manifest'])) {
          $manifest = json_decode(file_get_contents("$module_path/{$options['manifest']}"), TRUE);
          $asset_destination = trim($manifest[$asset_source] ?? '', '/');
          if (!empty($asset_destination)) {
            unset($options['manifest']);
            unset($libraries[$name]['js'][$asset_source]);
            $libraries[$name]['js'][$asset_destination] = $options;
          }
        }
      }
    }

    if (isset($library['css']['theme'])) {
      foreach ($library['css']['theme'] as $asset_source => $options) {
        if (isset($options['manifest'])) {
          $manifest = json_decode(file_get_contents("$module_path/{$options['manifest']}"), TRUE);
          $asset_destination = trim($manifest[$asset_source] ?? '', '/');
          if (!empty($asset_destination)) {
            unset($options['manifest']);
            unset($libraries[$name]['css']['theme'][$asset_source]);
            $libraries[$name]['css']['theme'][$asset_destination] = $options;
          }
        }
      }
    }
  }
}
?>

This allows you to define a manifest.json path which will be used when the caches get rebuilt. The libraries.yml looks something like this.

main:
  js:
    build/js/main.js: { manifest: 'build/manifest.json' }
  css:
    theme:
      build/css/main.css: { manifest: 'build/manifest.json' }
  dependencies:
    - core/jquery
    - core/drupal

admin-tools:
  js:
    build/js/admin-tools.js: { manifest: 'build/manifest.json' }
  css:
    theme:
      build/css/admin-tools.css: { manifest: 'build/manifest.json' }

Here is an example manifest.json

{
  "build/js/main.js": "/build/js/main.js",
  "build/js/admin-tools.js": "/build/js/admin-tools.js",
  "build/css/main.css": "/build/css/main.css",
  "build/css/admin-tools.css": "/build/css/admin-tools.css"
}