Issue #2377: Make importlib the implementation of __import__().

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__().

Python/freeze_importlib.py

@@ -0,0 +1,37 @@
+#! /usr/bin/env python
+"""Freeze importlib for use as the implementation of import."""
+import marshal
+
+
+header = """/* Auto-generated by Python/freeze_importlib.py */"""
+
+
+def main(input_path, output_path):
+    with open(input_path, 'r', encoding='utf-8') as input_file:
+        source = input_file.read()
+
+    code = compile(source, '<frozen importlib._bootstrap>', 'exec')
+
+    lines = [header]
+    lines.append('unsigned char _Py_M__importlib[] = {')
+    data = marshal.dumps(code)
+    # Code from Tools/freeze/makefreeze.py:writecode()
+    for i in range(0, len(data), 16):
+        line = ['    ']
+        for c in data[i:i+16]:
+            line.append('%d,' % c)
+        lines.append(''.join(line))
+    lines.append('};\n')
+    with open(output_path, 'w') as output_file:
+        output_file.write('\n'.join(lines))
+
+
+if __name__ == '__main__':
+    import sys
+
+    args = sys.argv[1:]
+    if len(args) != 2:
+        print('Need to specify input and output file paths', file=sys.stderr)
+        sys.exit(1)
+
+    main(*args)