You wouldn't. Well I feel like the current behavior isn't the behavior we used to have, that mIRC was looking for file in order to execute an alias, not that it would be first look into the file you are currently trying to call the alias from. I reported the bug in that sense.
Now it's likely that I assumed that behavior, where the current behavior has probably been there since ever.
However, there's still no way to accomplish what westor wanted to do, though that will probably lead to a feature suggestion.