gh-159963: Don't hold parent package import locks while a submodule executes (GH-159965)

Since gh-83065 (3.15.0), the first import of pkg.mod kept pkg's module
lock until pkg/mod.py had finished executing.  Threads that needed only
the finished pkg stayed blocked for that long, and concurrent first
imports could raise _DeadlockError or a spurious "partially initialized
module" ImportError, or hang, with no import cycle involved.

Parent packages are now imported, or waited for if another thread is
initializing them, before the submodule's module lock is taken, so only
that lock is held while the submodule executes.  The lock ordering
deadlock fixed by gh-83065 stays fixed.

importlib.import_module() now returns the partially initialized module
in a circular import split across threads, as the import statement
already did, instead of raising _DeadlockError.  That was long-standing
behavior rather than part of the 3.15.0 regression.