Fix pip config parsing for multiple extra-index-urls

When pip config contains multiple extra-index-urls, `pip config list`
outputs them separated by literal `\n` characters. The previous regex
only split on whitespace, so multiple URLs were treated as a single
mangled value. Split on literal `\n` as well.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: harkrish-1

metaflow/plugins/pypi/pip.py

@@ -324,7 +324,7 @@ def indices(self, prefix):
                 key, value = line.split("=", 1)
                 _, key = key.split(".")
                 if key in ("index-url", "extra-index-url"):
-                    values = map(lambda x: x.strip("'\""), re.split("\s+", value, re.M))
+                    values = map(lambda x: x.strip("'\""), re.split(r"\\n|\s+", value, re.M))
                     (indices if key == "index-url" else extra_indices).extend(values)
         except Exception:
             pass