Fix lookupCommand with multiple addon commands

Previously `lookupCommand` iterated through every addon with commands,
reassigning `addonCommand` for each one. As a result, having multiple
addons with commands installed would clobber lookup of all but the last
addon’s commands.

Now it breaks out of the loop as soon as it finds a matching command.
Merge pull request #2121 from jgwhite/lookup-command-patch

Fix lookupCommand with multiple addon commands