bpo-37641 preserve relative file location in embeddable zip (GH-14884)

Previously, pyc files in the embeddable distribution reported their
location as <build path>/<file stem>.py. This causes a little confusion
when interpreting stack traces as the file is in a (almost certainly)
incorrect location, and lacks the full relative path to Lib (e.g.
email/mime/image.py will only show image.py).

This change preserves the Lib relative location of the source file as a
path so that stack traces are (hopefully) less misleading and more
informative.

Co-Authored-By: Kyle Stanley <aeros167@gmail.com>