Incorrect success message when adding an alias without an @ symbol #5391

Bug Report

Describe the current, buggy behavior

Adding an alias but forgetting its leading @ symbol results in an invalid alias being added, and a success message being shown to the user.

Describe how other contributors can replicate this bug

• Run wp cli alias add hello --set-path=/ and observe a success message
• Run wp @hello help and observe the alias does not exist
• Run wp cli alias list and observe the alias is not listed
• Open your ~/wp-cli/config.yml file and observe that the alias was added without a leading @ symbol

Describe what you expect as the correct outcome

I would like the wp cli alias commands to work both with and without the leading @ symbol, with one being added as necessary behind the scenes.

The --grouping flag appears to be the only place in WP-CLI where the leading @ symbol is optional. Both of the following work:

• wp cli alias add @foo --grouping=bar,baz
• wp cli alias add @foo --grouping=@bar,@baz