Make Agg[T] an alias for Seq[T]

[lihaoyi]

Agg was a good idea at the time, but 7 years later I don't think the bespoke complexity is paying off. We should move all user facing APIs to Seq and make Agg an alias for backwards source compatibility

This is a bincompat breaking change and will need to go into 0.13.0

[lefou]

Agg ensured (silently) that there were no duplicate entries. When removing it, we should also revise other places like T.sources/Task.Sources or ivyDeps whether they should de-duplicate their inputs to avoid extra work.

[lolgab]

For source compatibility you can define a Agg object, so you can provide a when method if you want, it can be already deprecated if you want to remove it eventually, otherwise we need to deprecate it now in 0.12.x so people can start migrating away from it.