GH-83065: Fix import deadlock by implementing hierarchical module locking (GH-137196)

GH-83065: Fix import deadlock by implementing hierarchical module locking (GH-137196)

Make _find_and_load() acquire the module locks for the full
dotted-name chain (parent before child) when loading a nested module, so
both threads contend on the same first lock and serialise instead of
deadlocking.

When acquiring a parent's lock would itself deadlock with another thread
that is loading that parent (cross-package circular imports), the parent's
lock is skipped and the partially-initialised parent is accepted -- the
same policy _lock_unlock_module() already applies on the existing code
path -- so concurrent circular imports that worked before continue to work.