code: function find(id, type, sourcedir, options) {
    const { cache, fileExtensions, nativeModules } = options;
    const pkgDetails = pkg.getDetails(sourcedir, options);
    let filepath = isRelativeFilepath(id) ? path.join(sourcedir, id) : id;
  
    filepath = alias.resolve(filepath, pkgDetails && pkgDetails.aliases);
    if (filepath === false || nativeModules.includes(filepath)) return false;
  
    if (isAbsoluteFilepath(filepath)) {
      filepath = findFilepath(filepath, type, fileExtensions);
      filepath = alias.resolve(filepath, pkgDetails && pkgDetails.aliases);
      // File doesn't exist or is disabled
      if (filepath === '' || filepath === false) return filepath;
      // File found
      if (isAbsoluteFilepath(filepath)) {
        // Cache
        cache.setFile(
          {
            id: pkg.resolveId(pkgDetails, filepath),
            path: filepath,
            version: pkgDetails.version
          },
          versionDelimiter
        );
  
        return filepath;
      }
    }
  
    // Update id if it has been resolved as package
    if (!isFilepath(filepath)) id = filepath;
  
    // Search paths for matches
    pkgDetails.paths.some(sourcepath => {
      if (id && sourcedir != sourcepath) {
        let fp = path.join(sourcepath, id);
  
        fp = find(fp, type, fp, options);
        if (fp !== '') {
          filepath = fp;
          return true;
        }
        filepath = '';
      }
    });
  
    return filepath;
  }
docs: Find filepath for 'id' in 'sourcedir' directory
  @param {String} id
  @param {String} type
  @param {String} sourcedir
  @param {Object} options
  - {Boolean} browser
  - {ResolverCache} cache
  - {Object} fileExtensions
  - {Array} nativeModules
  - {Array} sources
  @returns {String|Boolean}