[GSoC] Better lazy_imports usability

Part1: Currently, lazy imports feels counter intuitive and confusing. It would be nice to improve the API to allow something like:

with tfds.core.lazy_imports():
  import some_module

It would patch the default module import:

• If the module is installed (e.g. tested with try/except ImportError): Import the module as standard
• If the import isn't installed: Replace the module by a dummy ModuleType which raise error when access attribute (use __getattr__):

with tfds.core.lazy_imports():
  import some_module

some_module.some_function()  # Raise the lazy import error (please install some_module)

Start by locating the tfds.core.lazy_imports API and its existing import-handling tests or entry points. Compare the requested context-manager behavior with the current implementation, including installed-module imports and the dummy ModuleType path. Done means missing modules raise the requested lazy-import error only when an attribute is accessed, while installed modules behave like standard imports.