bpo-38735: Don't fail when importing from / with sys.pycache_prefix set

[encukou]

This fixes the issue, but testing it would require putting a file in the root directory.

https://bugs.python.org/issue38735

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

You can test it without writing a file.

>>> from importlib.util import cache_from_source
>>> import os
>>> os.chdir('/')
>>> cache_from_source('spam.py')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    cache_from_source('spam.py')
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "<frozen importlib._bootstrap_external>", line 558, in cache_from_source
IndexError: string index out of range