docs: def parse_as_folder_reference(dep):
      """ See if a dependency reference refers to a folder path.
          If it does, return the folder path (which parses and
          resolves file:// urls in the process).
          If it doesn't, return None.
      """
      # Special case: pep508 urls
      if dep.find("@") > 0 and (
              (dep.find("@") < dep.find("/") or "/" not in dep) and
              (dep.find("@") < dep.find(":") or ":" not in dep)
              ):
          # This should be a 'pkgname @ https://...' style path, or
          # 'pkname @ /local/file/path'.
          return parse_as_folder_reference(dep.partition("@")[2].lstrip())
  
      # Check if this is either not an url, or a file URL:
      if dep.startswith(("/", "file://")) or (
              dep.find("/") > 0 and
              dep.find("://") < 0):
          if dep.startswith("file://"):
              dep = urlunquote(urlparse(dep).path)
          return dep
      return None