pkg/tool: add constraints for tasks

Updates #39

Change-Id: I9036a366a798cb2cd08d6e4695c1ab9e6d32da66
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1940
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/pkg/tool/cli/cli.cue b/pkg/tool/cli/cli.cue
index 986f43a..0318ff0 100644
--- a/pkg/tool/cli/cli.cue
+++ b/pkg/tool/cli/cli.cue
@@ -16,7 +16,7 @@
 
 // Print sends text to the stdout of the current process.
 Print: {
-	kind: "tool/cli.Print"
+	kind: *"tool/cli.Print" | "print" // for backwards compatibility
 
 	// text is the text to be printed.
 	text: string