create log file's base directory if it doesn't exist

pip/basecommand.py

@@ -184,6 +184,9 @@ def open_logfile(filename, mode='a'):
     the file to separate past activity from current activity.
     """
     filename = os.path.expanduser(filename)
+    dirname = os.path.dirname(filename)
+    if not os.path.exists(dirname):
+        os.makedirs(dirname)
     exists = os.path.exists(filename)
     log_fp = open(filename, mode)
     if exists: