`bundled_pip_wheel_list[0]: unbound variable` error if a broken Python install exists in app source

[edmorley]

Spotted via Honeycomb metrics and then reproduced locally...

If an app's source contains a file at .heroku/python/bin/python, then the build currently fails with:

-----> Using Python 3.9.20 specified in .python-version
-----> Using cached install of Python 3.9.20
/tmp/buildpack/lib/utils.sh: line 33: bundled_pip_wheel_list[0]: unbound variable

Apps should not have any contents at .heroku/python/ (it's an internal location used by the buildpack), but I'm guessing perhaps some users may have downloaded an app's slug to rescue the source from an app years ago (from a Python version that doesn't have ensurepip?), and then extracted that directly into their Git repo without removing the Python binaries?

GUS-W-17386432.

[edmorley]

The error message here has been improved in #1720 / #1724.