Use scala.collection.Seq explicitly instead of scala.Seq alias

scala.Seq is widely used in the code, and is an alias for scala.collection.Seq in Scala 2.12. It will become an alias for scala.collection.immutable.Seq in Scala 2.13. In many cases, this will be fine, as Spark users using Scala 2.13 will also have this changed alias. In some cases it may be undesirable, as it will cause some code to compile in 2.12 but not in 2.13. In some cases, making the type scala.collection.Seq explicit so that it doesn't vary can help avoid this, so that Spark apps might cross-compile for 2.12 and 2.13 with the same source.