addon/info: Demote circular dependency errors to info loglevel

It turns out this is actually very useful according to a certain add-on
author who is definitely not me.

For example: have Campaign Foo and Music Pack Foo. Music Pack Foo can
depend upon Campaign Foo so a player attempting to download it is
suggested to download Campaign Foo as well since it's entirely useless
otherwise. Meanwhile, Campaign Foo depends upon Music Pack Foo in order
to be able to use some custom music in the campaign proper.