debugmsg to DebugLog for "lua replaces iuse"

Because we don't want to be bitching at the player when this happens, but it might be useful information. Closes #9675.

Note: this still leaves in a rather serious issue with iuses sticking around after they should've been cleared -- this is detailed in the discussion of the linked issue. It is potentially possible (and even likely) that an iuse defined in a mod would stick around in a world where it wouldn't exist if that world had been loaded first. @BevapDin suggests clearing the iuse_function_list and rebuilding it on every game unload and reload, and I would have to agree -- I just don't feel comfortable writing that.