stdlib: adapted lib to new input spec

Signed-off-by: Sam Alba <sam.alba@gmail.com>

stdlib/dagger/dagger.cue

@@ -7,7 +7,9 @@ import (
 // An artifact such as source code checkout, container image, binary archive...
 // May be passed as user input, or computed by a buildkit pipeline
 
+// FIXME (perf). See https://github.com/dagger/dagger/issues/445
 #Artifact: {
+	@dagger(artifact)
 	#up: [...op.#Op]
 	_
 	...
@@ -17,5 +19,6 @@ import (
 // FIXME: currently aliased as a string to mark secrets
 // this requires proper support.
 #Secret: {
+	@dagger(secret)
 	string | bytes
 }