> There should be an easy way for a module to declare itself to be lazy loaded.

It can just implement lazy loading itself today, by using module-level __getattr__ (https://docs.python.org/3/reference/datamodel.html#customizi...) to overwrite itself with a private implementation module at the appropriate time. Something like:

  # foo.py
  def __getattr__(name):
      # clean up the lazy loader before loading
      # this way it's cleaned up if the implementation doesn't replace it,
      # and not scrubbed if it does
      global __getattr__
      del __getattr__
      import sys
      self = sys.modules[__name__]
      from . import _foo
      # "star-import" by adding names that show up in __dir__
      self.__dict__.update(**{k: getattr(_foo, k) for k in _foo.__dir__()})
      # On future attribute lookups, everything will be in place.
      # But this time, we need to delegate to the normal lookup explicitly
      return getattr(self, name)

Genericizing this is left as an exercise.