Full ESM seems to work only in 11ty v3, which is still alpha

.eleventy.js

@@ -5,8 +5,8 @@
  * License: Attribution-ShareAlike 4.0 Unported
  *          http://creativecommons.org/licenses/by-sa/4.0/
  */
-import facebook from './lib/facebook-meta';
-import flickrImageList from './lib/flickr-image-list';
+import facebook from './lib/facebook-meta.js';
+import flickrImageList from './lib/flickr-image-list.js';
 
 /** @param {import("@11ty/eleventy").UserConfig} eleventyConfig */
 export default function(eleventyConfig) {

lib/flickr-image-list.js

@@ -7,6 +7,7 @@
  */
 import fs from 'node:fs';
 import path from 'node:path';
+import url from 'node:url';
 
 // If any of the files in 'content/*/*.md' has changed, update the whole cache.
 const FLICKR_MATCHER = /(https:\/\/.+\.static\.?flickr\.com\S+\.jpg)/gu;
@@ -56,6 +57,7 @@ export const findImages = function findImages(dirpath) {
  * @returns {Array.<string>} List of images
  */
 export default function flickrImageList() {
+  const __dirname = url.fileURLToPath(new URL('.', import.meta.url));
 
   // Loop all Markdown files under content/*/
   const dir = path.join(__dirname, '../content/');