Fix custom pmaports dir (pmbootstrap --args)
The aports argument is a list of paths now, so iterate over each of them when checking if they exist.
Fix for:
INFO:root:ERROR: 'list' object has no attribute 'exists'
somewhat related: I think before we release pmbootstrap 3.0.0 it would be a good time to finally rename --aports to --pmaports (#1906). but let's get this fix merged first.