code: def add_source_override(self, fullname, path, source, is_pkg):
          
          self._found_cache[fullname] = (path, source, is_pkg)
docs: Explicitly install a source cache entry, preventing usual lookup
          methods from being used.
  
          Beware the value of `path` is critical when `is_pkg` is specified,
          since it directs where submodules are searched for.
  
          :param str fullname:
              Name of the module to override.
          :param str path:
              Module's path as it will appear in the cache.
          :param bytes source:
              Module source code as a bytestring.
          :param bool is_pkg:
              :data:`True` if the module is a package.