package/python3: ensure pyc embed rooted paths

To report usable tracebacks, pyc files embed the path of the original py
files, so that users can more easily try and debug the reported issue.

We generate the pyc files by calling the python3-supplied compileall
script, to scan the directory where python modules are installed. Since
this is done on the build machine, we tell compileall.py to strip away
the TARGET_DIR prefix, as that has no meaning at runtime.

However, compileall.py forgets [0] to keep a leading / in the front of
the paths, thus generating non-rooted paths., e.g.:
     /path/buildroot.ouput/targt/usr/lib/python3.10/argparse.py
gets embedded as:
     usr/lib/python3.10/argparse.py

This is a bit confusing but, as far as we could see, should be mostly be
used for display purposes in tracebacks, and does not seem to impact
actual functionality.

We fix that by instructing compileall.py that the embedded paths should
be rooted to / which generates proper paths in tracebacks.

And alternate solution would be to swith gears, and tell compileall.py
exactly the resulting runtime "base" directory, which replaces the
stripping and prefixing; i.e. it's either:
    -s $(TARGET_DIR) -p /
or
    -d /usr/lib/python$(PYTHON3_MAJOR_VERSION)

We choose to keep the first solution, because that is semantically what
we really want to do: to strip the leading build-time path, rather than
to force anything.

Note: the python test-suite was executed with both solutions (in a
pyc-only setup), and the results were exactly the same; so in practice,
-d or -s+-p yield the same results.

Many thanks go to Vincent for reporting the issue and suggesting the
solutions.

[0] Not sure whether this is a bug or a feature...

Reported-by: Vincent Fazio <vfazio@xes-inc.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>