bpo-39769: Fix compileall ddir for subpkgs. (GH-18676)

Fix compileall.compile_dir() ddir= behavior on sub-packages.

Fixes compileall.compile_dir's ddir parameter and compileall command
line flag `-d` to no longer write the wrong pathname to the generated
pyc file for submodules beneath the root of the directory tree being
compiled.  This fixes a regression introduced with Python 3.5.

Also marks the _new_ in 3.9 from PR #16012 parameters to compile_dir as keyword only (as that is the only way they will be used) and fixes an omission of them in one place from the docs.

Lib/test/test_importlib/util.py

@@ -7,6 +7,7 @@ import importlib
 from importlib import machinery, util, invalidate_caches
 from importlib.abc import ResourceReader
 import io
+import marshal
 import os
 import os.path
 from pathlib import Path, PurePath
@@ -118,6 +119,16 @@ def submodule(parent, name, pkg_dir, content=''):
     return '{}.{}'.format(parent, name), path
 
 
+def get_code_from_pyc(pyc_path):
+    """Reads a pyc file and returns the unmarshalled code object within.
+
+    No header validation is performed.
+    """
+    with open(pyc_path, 'rb') as pyc_f:
+        pyc_f.seek(16)
+        return marshal.load(pyc_f)
+
+
 @contextlib.contextmanager
 def uncache(*names):
     """Uncache a module from sys.modules.