External libraries with no depedendencies create extra aggregates

This isn't fixable in itself because potentially a file in the aggregate could depend on the external library. Also less likely but possible, the external library could depend on something in the aggregate.

However if we do 📌 Deprecate support for per-file weight in libraries API Active then we could look at additionally ordering libraries with aggregates either first or last before other ordering takes place, then they would be either at the beginning or the end and would split aggregates less often.