Force native event handler to be registered for activation commands

We use onWillDispatch internally to manage activation commands,
activating packages *before* the activation command starts dispatching.
This means we need to explicitly tell the command registry to subscribe
to the command in question.

src/package.coffee

@@ -335,6 +335,7 @@ class Package
     for selector, commands of @getActivationCommands()
       for command in commands
         do (selector, command) =>
+          atom.commands.commandRegistered(command)
           @activationCommandSubscriptions.add(atom.commands.onWillDispatch (event) =>
             return unless event.type is command
             currentTarget = event.target