Avoid infinite loop in path check

[stratofax]

If the path to the BallotMaker package doesn't include the word "ballotmaker" the program will run an infinite loop. Add a test to break this loop.

From @ion-oset

Suggestion:

  assert path_finder != "/", "Path search reached the file system root."

Or use:

python/importlib_resources

[ion-oset]

In the immediate term stick to the assertion to avoid the infinite loop. When there's time the correct answer is importlib.resources.files which is explicitly intended to allow access to files from within a package across platforms. This API is available starting from Python 3.9, but exists as a backport using the PyPI package importlib-resources.

See:

• Migrating from Legacy for details.
• NIST Data Models test setup for how it can work in practice.

I'm happy to pick the PR up for adding importlib-resources when we decide what milestone it's on.