Issue #25771: Tweak ValueError message when package isn't specified

for importlib.util.resolve_name() but is needed.

Thanks to Martin Panter for the bug report.

Lib/importlib/util.py

@@ -22,8 +22,8 @@ def resolve_name(name, package):
     if not name.startswith('.'):
         return name
     elif not package:
-        raise ValueError('{!r} is not a relative name '
-                         '(no leading dot)'.format(name))
+        raise ValueError(f'no package specified for {repr(name)} '
+                         '(required for relative module names)')
     level = 0
     for character in name:
         if character != '.':