bug#53258: Python unable to find modules within a Singularity container created with guix pack

[Josselin Poiret]

Hello everyone,

After the IRC investigation, it turned out the use of abspath in
Lib/site.py was in fact the culprit. Replacing all its uses with
realpath and doing `./pre-inst-env guix pack -RR python python-numpy -S
/bin=bin -S /etc=etc` did in fact resolve the issue!

I've raised the issue upstream at [1].  The fix itself is pretty simple,
but it's theoretically a breaking change, so we'll see how Python
maintainers want to move forward with it.  We could backport this fix
for the next core-updates cycle, whether or not it gets accepted
upstream, and we will also need to modify sitecustomize.py as well to
call os.path.realpath on python_site.

Best,
-- 
Josselin Poiret