gh-154561: Check module dict after trying to load submodule

[johnslavik]

Locally I bumped the test to use 100 threads for higher failure reproducibility:

diff --git a/Lib/test/test_lazy_import/__init__.py b/Lib/test/test_lazy_import/__init__.py
index 899ecec1ba2..3d44d2d04c9 100644
--- a/Lib/test/test_lazy_import/__init__.py
+++ b/Lib/test/test_lazy_import/__init__.py
@@ -1794,7 +1794,7 @@ def test_concurrent_lazy_import_reification(self):
         """Multiple threads racing to reify the same lazy import should succeed."""
         from test.test_lazy_import.data import basic_unused
 
-        num_threads = 10
+        num_threads = 100
         results = [None] * num_threads
         errors = []
         barrier = threading.Barrier(num_threads)

I cannot reproduce failures after the proposed change: after trying to load the submodule, we re-check the module dict in case a different thread beat us to it, in which case the attribute can be returned directly.

• Issue: test_concurrent_lazy_import_reification is flaky #154561