mpl: evaluate impact of weight connections in designs with PADs

[AcKoucher]

Description

Nowadays nets with IO Pins (either connected or not to PADs) are weighted differently than others.

Suggested Solution

Investigate the impact of neutralizing that difference depending on the type of IO.

Additional Context

No response

[AcKoucher]

Some investigation showed that we already have the desired behavior. We only weight the IO clusters' nets differently when they're groups of unplaced pins. This investigation lead to find out a problem with our current approach of how we handle pad instances in mpl. See #6894.