Migrate dagger.#Secret and dagger.#Stream to new format

- Refactored to keep every transformation of built-in types (e.g. FS,
  Secret, etc) to/from CUE in the same place (plancontext)
- dagger.#Service and dagger.#Secret are now following the new FS-like format
  (e.g. `_service: id: string`)
- Backward compatibility
- dagger.#Stream is now an alias for dagger.#Service

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

cmd/dagger/cmd/common/common.go

@@ -88,7 +88,11 @@ func FormatValue(val *compiler.Value) string {
 	if val.HasAttr("artifact") {
 		return "dagger.#Artifact"
 	}
-	if val.HasAttr("secret") {
+
+	if val.LookupPath(cue.MakePath(
+		cue.Hid("_secret", "alpha.dagger.io/dagger"),
+		cue.Str("id"),
+	)).Exists() {
 		return "dagger.#Secret"
 	}
 	if val.IsConcreteR() != nil {