[dev.typeparams] cmd/compile/internal/types2: move newTypeSet function into typeset.go

No functional changes except for import declaration and comment
adjustments.

Change-Id: I75fb5edba8b89a5aad7c9b4ddb427c201265def0
Reviewed-on: https://go-review.googlesource.com/c/go/+/331515


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>