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>

tests/compute/secrets/simple/simple.cue

@@ -17,7 +17,6 @@ TestSecrets: #up: [
 
 	op.#Exec & {
 		mount: "/secret": secret: mySecret
-		env: PLAIN: mySecret.id
 		args: [
 			"/bin/bash",
 			"--noprofile",
@@ -27,7 +26,6 @@ TestSecrets: #up: [
 			"-c",
 			#"""
 				test "$(cat /secret)" = "SecretValue"
-				test "$PLAIN" != "SecretValue"
 				"""#,
 		]
 	},