fix crash resolving cross-refs to suppressed modules

[cburroughs]

When a stub imports from a module without stubs (e.g., protobuf stubs importing from google.protobuf when protobuf stubs aren't installed), the import is suppressed but a cross-ref is cached. On cache reload, resolving the cross-ref fails along the lines of:

AssertionError: Cannot find module for google

This occurs because the cross-ref points to a module that was suppressed and therefore never loaded into manager.modules.

The fix passes the set of suppressed modules to fixup_module() and checks if a cross-ref points to a suppressed module before attempting lookup. If so, a placeholder node is used instead.

Before this change on my test setup (see #20655) with 128 shards iterating through different dependency combination, 29 crashed with this error. With the change 0 do so. The reproduction at Qiskit/qiskit-serverless#1787 also passes for me with this change.

Fixes #16214

Disclaimer: I don't have prior familiarity with the mypy codebase and this change was heavily LLM assisted.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[jonaslb]

Suggested change

	parts = name.split(".")
	for i in range(len(parts), 0, -1):
	prefix = ".".join(parts[:i])
	if prefix in suppressed:
	return True
	return False
	idx = 0
	while True:
	dot = name.find(".", idx)
	if dot == -1:
	return name in suppressed
	prefix = name[:dot]
	if prefix in suppressed:
	return True
	idx = dot + 1

(I'm not a mypy maintainer, I just noticed that you are building a new string a number of times, which would seem unnecessary, particularly for very.long.module.names.like.this._and.stuff)