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.