Separate weights for the two pallet_collective instances

[clangenb]

We now use the same weights for both pallet instances. However, we already benchmark both, and they somehow write into the same file.

This is different in Polkadot, where they have different files. On first sight we seem to be doing the benchmark similar to Polkadot. So we have to research a bit to find out what we need to do.

See how polkadot gets the weight: https://github.com/paritytech/polkadot/blob/8ac29d3846075ec98fbb386d4aa829687df97efe/runtime/polkadot/src/lib.rs#L748