load C extensions of external packages correctly

At present, all C extension modules loaded in the APE Python are
built-in modules -- they are compiled into the interpreter instead of
being separate shared objects in some folder.

In the Python3.6 stdlib, the modules usually follow a naming convention
like: X imports a module called _X, which is built from Modules/_X.c.
(example: json imports _json, which is built from Modules/_json.c).

However, external Python packages follow a different naming convention:
X imports a module called ._Y, which is available as a shared object
located in the same directory as X.

(example: markupsafe imports from ._speedups, expecting a _speedups.so
to be available within the markupsafe folder).

This change makes the interpreter load the required module as a builtin
(if present) if the requisite shared object/python file is not found.
The benefit of this is that the external packages can keep the same
naming convention for their C extensions, and once built, the APE will
load those extensions without error.

third_party/python/Lib/importlib/_bootstrap.py

@@ -949,6 +949,19 @@ def _find_and_load_unlocked(name, import_):
             msg = (_ERR_MSG + '; {!r} is not a package').format(name, parent)
             raise ModuleNotFoundError(msg, name=name) from None
     spec = _find_spec(name, path)
+    if spec is None and name in sys.builtin_module_names:
+        # If this module is a C extension, the interpreter
+        # expects it to be a shared object located in path,
+        # and returns spec is None because it was not found.
+        #
+        # however, if it is a C extension, we can check if it
+        # is available using sys.builtin_module_names,
+        # because the APE is statically compiled.
+        #
+        # if the module is present as a builtin, we call
+        # BuiltinImporter with the full name (and no path)
+        # to create the module spec correctly.
+        spec = BuiltinImporter.find_spec(name)
     if spec is None:
         raise ModuleNotFoundError(_ERR_MSG.format(name), name=name)
     else: