Make importlib compatible with __import__ by "fixing" code.co_filename

paths.

__import__ does a little trick when importing from bytecode by
back-patching the co_filename paths to point to the file location
where the code object was loaded from, *not* where the code object was
originally created. This allows co_filename to point to a valid path.
Problem is that co_filename is immutable from Python, so a private
function -- imp._fix_co_filename() -- had to be introduced in order to
get things working properly. Originally the plan was to add a file
argument to marshal.loads(), but that failed as the algorithm used by
__import__ is not fully recursive as one might expect, so to be fully
backwards-compatible the code used by __import__ needed to be exposed.

This closes issue #6811 by taking a different approach than outlined
in the issue.

Lib/importlib/_bootstrap.py

@@ -404,6 +404,7 @@ class SourceLoader(_LoaderBasics):
                     else:
                         found = marshal.loads(bytes_data)
                         if isinstance(found, code_type):
+                            imp._fix_co_filename(found, source_path)
                             return found
                         else:
                             msg = "Non-code object in {}"