static __normalizeMainEntry(mainEntry) {
    if (mainEntry.startsWith('/')) {
      return /** @type {PathRelativeFromProjectRoot} */ (`.${mainEntry}`);
    }
    if (!mainEntry.startsWith('.')) {
      return `./${mainEntry}`;
    }
    return /** @type {PathRelativeFromProjectRoot} */ (mainEntry);
  }
From 'main/file.js' or '/main/file.js' to './main/file.js'
@param {string} mainEntry
@returns {PathRelativeFromProjectRoot}