importlib._bootstrap is now frozen into Python/importlib.h and stored
as _frozen_importlib in sys.modules. Py_Initialize() loads the frozen
code along with sys and imp and then uses _frozen_importlib._install()
to set builtins.__import__() w/ _frozen_importlib.__import__().
case-sensitive filesystems -- which is not the default case. Along the way also
fixed the skipping of tests when sys.dont_write_bytecode is true.
Closes issue #5442 again.