Hooking Metacello load

We are further advancing with Iceberg and its metacello integration here.

Now iceberg is not package based anymore (though packages are still there :)).

The thing is making iceberg package-independent required us to decouple us from monticello. And well, our metacello integration is aching a bit.

We already have a place to hook into metacello using a MetacelloPlatform subclass. However, we would like to control also how packages are loaded into the system. Right now metacello uses gofer and hooks into it by creating a `MetacelloGoferLoad` instance in a couple of places. What do you think if we move that to a hook in the MetacelloPlatform?

I'm thinking in something like:

MetacelloPlatform >> goferLoadClass

  ^ MetacelloGoferLoad

or

and then inside metacello you can still do

MetacelloPlatform current goferLoadClass on: ....

or

That shouldn't break any external users and then on our side we could

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