bpo-38735: Don't crash when importing from / with pycache_prefix set

Author: encukou

Lib/importlib/_bootstrap_external.py

@@ -465,9 +465,9 @@ def cache_from_source(path, debug_override=None, *, optimization=None):
             head = _path_join(_os.getcwd(), head)
 
         # Strip initial drive from a Windows path. We know we have an absolute
-        # path here, so the second part of the check rules out a POSIX path that
+        # path here, so the first part of the check rules out a POSIX path that
         # happens to contain a colon at the second character.
-        if head[1] == ':' and head[0] not in path_separators:
+        if head[0] not in path_separators and head[1] == ':':
             head = head[2:]
 
         # Strip initial path separator from `head` to complete the conversion