(cherry picked from commit e9d9494d6b2a5e0c2d48d22c7f0d5e95504b4f7e)

instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.
(cherry picked from commit 8a9cd20edca7d01b68292036029ae3735ce65edd)

* Rewrite importlib _get_module_lock(): it is now responsible to hold
  the imp lock directly.
* _find_and_load() now holds the module lock to check if name is in
  sys.modules to prevent a race condition
(cherry picked from commit 4f9a446f3fb42f800e73cd9414dd1eccb3ca4fa7)

(cherry picked from commit b4baacee1adc06edbe30ac7574d17a8cd168e2e0)

In rare circumstances PyImport_Import() could return NULL without raising
an error.
(cherry picked from commit 145541c)

The latter function is more readable, faster and doesn't raise exceptions.

The latter function is more readable, faster and doesn't raise exceptions.

PyModule_GetDef() can return NULL. Let's check the return value properly
like in the other five cases.

CID 1299590