bpo-30891: Fix again importlib _find_and_load() (#2665) (#2801)

bpo-30891: Fix again importlib _find_and_load() (#2665) (#2801)

Use sys.modules.get() in the "with _ModuleLockManager(name):" block
to protect the dictionary key with the module lock and use an atomic
get to prevent race condition.

Remove also _bootstrap._POPULATE since it was unused
(_bootstrap_external now has its own _POPULATE object), add a new
_SENTINEL object instead.
(cherry picked from commit e72b1359f81d1dd42bd8a5c5cc2b3928b74f8023)