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To respond to Nick's "yes, there are two copies of importlib._bootstrap" leanings, distutils2 has actually run into issues with this because they initially made some assumptions about consistency in what importlib returned vs. what import does (Arfrever can explain better than I can since he keeps pointing it out to me =).
If using _frozen_importlib to bootstrap in importlib._bootstrap is looking bad, then I'm fine w/ simply having the tests for importlib and imp use importlib._bootstrap and otherwise just use _frozen_importlib for everything else since I have tried to be diligent to add any and all import-related tests to importlib. Except while developing the code should be exactly the same so hiding the details really won't make much of a difference in the very common case.
If we go this route, though, then we really should take this time to do a proper context manager/decorator/whatever that covers all import state (including uncaching modules and sys.path_importer_cache) that we might care about and put the solution into test.support (what issue #14715 is asking for and I think is reasonable). We should also then add to regrtest detection of stuff left in sys.path_importer_cache or sys.modules that do not come from _frozen_importlib (which should help with the sporadic test_imp failure).