docs: def norm_proj_path(path, build_module):
      """Return a normalized path for the `path` observed in `build_module`.
  
      The normalized path is "normalized" (in the `os.path.normpath` sense),
      relative from the project root directory, and OS-native.
  
      Supports making references from project root directory by prefixing the
      path with "//".
  
      :raises ValueError: If path references outside the project sandbox.
      """
      if path == '//':
          return ''
  
      if path.startswith('//'):
          norm = normpath(path[2:])
          if norm[0] in ('.', '/', '\\'):
              raise ValueError("Invalid path: `{}'".format(path))
          return norm
  
      if path.startswith('/'):
          raise ValueError("Invalid path: `{}' - use '//' to start from "
                           "project root".format(path))
  
      if build_module == '//':
          build_module = ''
      norm = normpath(join(build_module, path))
      if norm.startswith('..'):
          raise ValueError(
              "Invalid path `{}' - must remain inside project sandbox"
              .format(path))
      return norm.strip('.')