GH-134872: add ModuleNotFoundError suggestions

[FFY00]

Needs GH-139900

• Issue: Add import suggestions for ModuleNotFoundError #134872

---

📚 Documentation preview 📚: https://cpython-previews--142512.org.readthedocs.build/

[FFY00]

@Locked-chess-official, this draft PR includes the changes from GH-139900, introducing the .discover() method, and my initial proposed traceback implementation.

I think it should address the concerns you raised in GH-139899, could you give it a look?

[FFY00]

Right now it doesn't special-case relative imports, but the message is still helpful.

Python 3.15.0a2+ free-threading build (heads/gh-139899:6816705de0c, Dec 10 2025, 12:54:51) [GCC 15.2.1 20251112] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import aa
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import aa
  File "/home/anubis/git/cpython/test-cases/aa/__init__.py", line 1, in <module>
    from .bb import *
ModuleNotFoundError: No module named 'aa.bb'. Did you mean: 'aa.bbb'?

I think it should be alright for an initial implementation, and then we can improve — just trying to make the scope smaller to hopefully guarantee this proposal doesn't lose momentum and ends up stalled.